Under Philippine law, the sanctity of marriage is protected by specific property regimes and moral codes. When a married individual uses marital funds to purchase property for a "mistress" (concubine or illicit partner), the law provides the legitimate spouse with several avenues to recover those assets.

The recovery process primarily hinges on the nature of the property regime governing the marriage and the nullity of donations made in violation of public policy.

1. The Governing Property Regime

The ability to recover property depends on whether the marriage is governed by the Absolute Community of Property (ACP) or the Conjugal Partnership of Gains (CPG).

ACP (Default for marriages after Aug 3, 1988): All property owned by the spouses at the time of marriage or acquired thereafter is owned jointly.
CPG (Default for marriages before Aug 3, 1988): Only the proceeds, fruits, and income from separate properties and everything acquired through effort during the marriage belong to the partnership.

In both regimes, any property acquired during the marriage is presumed to be conjugal or community property. If a spouse uses these funds to buy a house or car for a mistress, they are essentially disposing of assets that do not belong to them alone.

2. The Rule on Prohibited Donations

The most direct legal weapon is Article 87 of the Family Code, which states:

"Every donation or grant of gratuitous advantage, direct or indirect, between the spouses during the marriage shall be void, except moderate gifts... The same shall apply to parties living together as husband and wife without a valid marriage."

Furthermore, Article 739 of the Civil Code declares donations made between persons guilty of adultery or concubinage at the time of the donation as void ab initio (void from the beginning).

Key takeaway: A "gift" of property to a mistress is legally non-existent. Because the donation is void, ownership never actually transferred to the mistress, and the property remains part of the marital estate.

3. Legal Actions for Recovery

A. Action for Declaration of Nullity of Title/Sale

If the property was registered under the mistress's name, the legitimate spouse can file a civil case to declare the transfer void.

The Theory: Since the "donation" is void under Article 739, any title issued based on that donation is also void.
The Result: The court orders the cancellation of the mistress’s title and the issuance of a new one in the name of the conjugal partnership.

B. Judicial Reconveyance

This is a remedy used when property has been wrongfully registered in the name of another. The legitimate spouse asks the court to "reconvey" or return the property to the true owner (the marital partnership) because the mistress has no legal right to hold it.

C. Claim for Reimbursement (During Liquidation)

If the property cannot be physically recovered (e.g., it was sold to an innocent third party), the aggrieved spouse can demand that the value of the property be deducted from the share of the erring spouse during the eventual dissolution or liquidation of the marriage assets.

4. Relevant Jurisprudence: The "Twin" Protections

The Philippine Supreme Court has consistently ruled in favor of the legitimate family in these scenarios:

Impropriety of "Co-ownership" with a Mistress: Under Article 148 of the Family Code, in a relationship where one party is married, only the money/property they actually contributed is theirs. If the mistress cannot prove she earned the money to buy the property, it is presumed to have come from the married man’s marital funds.
Lack of Consent: Under both CPG and ACP, the sale or encumbrance of conjugal property requires the written consent of the other spouse. A purchase made for a mistress using marital funds is a "disposition without consent," which is generally voidable or void.

5. Practical Evidentiary Requirements

To succeed in recovering the property, the legitimate spouse must usually prove:

The Marriage: A valid marriage certificate.
The Illicit Relationship: Evidence of the concubinage or adultery (to trigger Article 739).
Source of Funds: Proof that marital funds (salaries, business income, or savings) were used to purchase the asset.
The Transaction: Deeds of sale, registration documents, or receipts naming the mistress as the buyer/owner.

6. The "Innocent Third Party" Complication

Recovery becomes difficult if the mistress has already sold the property to a "Buyer in Good Faith." If a third party bought the property without knowing it was purchased with stolen marital funds, the law may protect the buyer's title. In this case, the legitimate spouse’s remedy shifts to a personal action for damages against the erring spouse and the mistress.