The matrimonial investigation records, or diligencias matrimoniales, are part of the McPherson Collection at the Claremont Colleges Digital Collection. This material was collected originally by William F. McPherson, an Orange County California rancher, scholar and collector. The records, predominantly from the Mission San Gabriel (other California missions are also represented), consist of notarized interviews with couples requesting marriage in the Roman Catholic Church during the period 1788-1861. According to the description on line, "The purpose of marriage investigations was to prove that the parties were free to marry." These digital images are scans of the original documents and are in Spanish. So, you may need the help of someone you know that can read Spanish. As with most romance languages, you can figure out the surnames and many of the words because of the similarities. So, finding specific family surnames should be reasonably easy.
When you get to the site, just click on the large image on the left side of the scrren. That will take you to the images and descriptive summaries.