Commingling is the mixing of separate property assets with community property assets or vice versa. Property that is commingled is presumed to be community property. The presumption can be rebutted by tracing or following the money trail and providing proof of the separate claim. Proof of a separate claim could be demonstrated by bank account statements, real estate purchase agreements and deeds. If the tracing successfully proves that the source of the funds was only from a separate property source, the property that was commingled becomes uncommingled. If funds have been so commingled that it is virtually impossible to trace them back to their source, the funds are entirely community property.
Pursuant to FC 2581, property that is acquired during marriage in joint form is presumed to be community property. The presumption affects the burden of proof and can be rebutted by a clear statement in the deed or other documentary evidence of title by which the property is acquired or proof that the parties made a written agreement providing that the property is separate property.”
[LW Greenberg, California Family Law]