In the Philippines, infidelity is not merely a private grievance but a criminal offense. Under the Revised Penal Code (RPC), marital infidelity is prosecuted as either Adultery or Concubinage, depending on which spouse committed the act. For marriages involving foreigners or those celebrated abroad, the legal landscape becomes more complex due to the interplay of Philippine criminal law and private international law.

1. Defining the Crimes: Adultery vs. Concubinage

Philippine law maintains a distinction between the crimes based on the gender of the offending spouse.

Adultery (Article 333, RPC)

Adultery is committed by a married woman who has sexual intercourse with a man not her husband, and by the man who has carnal knowledge of her, knowing her to be married.

Nature of Proof: A single act of sexual intercourse is sufficient to constitute adultery.
Penalty: Prision correccional in its medium and maximum periods.

Concubinage (Article 334, RPC)

Concubinage is committed by a married man under any of the following three specific circumstances:

Keeping a mistress in the conjugal dwelling.
Having sexual intercourse under scandalous circumstances.
Cohabiting with a woman who is not his wife in any other place.

Nature of Proof: It is significantly harder to prove than adultery because it requires proof of "cohabitation" or "scandalous circumstances," not just a one-time sexual encounter.
Penalty: The husband faces prision correccional in its minimum and medium periods, while the concubine faces destierro (banishment).

2. Applicability to Foreign Marriages

The Philippines adheres to the Nationality Principle (Article 15, Civil Code), which states that laws relating to family rights and duties, status, and legal capacity of persons are binding upon citizens of the Philippines, even though living abroad.

If one spouse is Filipino

If a Filipino citizen is married to a foreigner (regardless of where the marriage was celebrated), the Filipino spouse remains subject to Philippine laws on marital fidelity.

Case Scenario: If a Filipino wife commits adultery abroad with a foreigner, she may still be prosecuted under Philippine law if she returns to the Philippines, provided the elements of the crime can be proven.
The Foreign Spouse: A foreign spouse can file these criminal charges in the Philippines against their Filipino spouse if the illicit acts occurred within Philippine territory.

If both spouses are Foreigners

Generally, Philippine criminal courts exercise jurisdiction over crimes committed within Philippine territory. If two foreigners are residing in the Philippines and one commits an act of infidelity here, the aggrieved spouse may technically file a complaint. However, the legal standing often depends on the recognition of their marriage under Philippine law.

3. The Impact of Foreign Divorce

A critical factor in filing for Adultery or Concubinage is the existence of a valid marriage.

Article 26 of the Family Code

Under the second paragraph of Article 26, if a Filipino is married to a foreigner and the foreigner subsequently obtains a valid foreign divorce that capacity the foreigner to remarry, the Filipino spouse shall likewise have the capacity to remarry under Philippine law.

Loss of Legal Standing: If the foreign spouse has already obtained a valid divorce abroad, they may lose the "legal standing" to file criminal charges for adultery or concubinage in the Philippines. This is because the law views the marital bond as severed; the "offended party" is no longer a "spouse" in the eyes of the law.
The "Clean Hands" Doctrine: If the person filing the case is the one who initiated the divorce, Philippine courts (following the precedent in Republic vs. Orbecido) generally do not allow that person to later claim the benefits of a marriage they chose to dissolve.

4. Requirements and Procedure for Filing

To initiate a criminal action for Adultery or Concubinage, the following procedural rules apply:

Private Crimes

These are "private crimes," meaning they can only be prosecuted upon a complaint filed by the offended party (the spouse). The state cannot initiate the prosecution on its own.

The Inclusion of the Accomplice

The offended party cannot file a case against only one of the guilty parties. The complaint must include both the offending spouse and the paramour/concubine, if both are alive.

Legal Bars to Filing

A case will be dismissed if:

Consent: The offended spouse consented to the infidelity.
Pardon: The offended spouse has expressly or impliedly pardoned the offending parties. Continued cohabitation after knowledge of the infidelity is often considered an implied pardon.

Step-by-Step Filing Process:

Preparation of Complaint-Affidavit: Detailed sworn statement and gathering of evidence (photos, hotel receipts, witness testimonies, birth certificates of illegitimate children).
Preliminary Investigation: Filing the complaint with the Office of the City or Provincial Prosecutor where the crime was committed.
Resolution: The Prosecutor determines if there is "probable cause."
Trial: If probable cause is found, an "Information" is filed in court, and a warrant of arrest is issued.

5. Jurisdictional Limitations

Criminal law in the Philippines is territorial.

If the acts of sexual intercourse or cohabitation occurred entirely outside of the Philippines, Philippine courts generally lack jurisdiction to try the criminal case, even if the parties are Filipino.
Criminal cases for Adultery or Concubinage must be filed in the municipality or city where the illicit act—or any of its essential ingredients—took place.

6. Summary Table: Comparison

Feature	Adultery (Wife)	Concubinage (Husband)
Who is Charged	Wife and her lover	Husband and his mistress
Required Proof	Single act of intercourse	Cohabitation, scandal, or keeping in home
Penalty for Spouse	Imprisonment (Medium/Max)	Imprisonment (Min/Medium)
Penalty for Partner	Imprisonment	Destierro (Banishment)
Foreign Divorce	Barred if divorce is valid	Barred if divorce is valid