Nonstochiometric mixed oxides are often of interest for their novel catalytic or material properties yet are often difficult to synthesize in easily characterizable forms. We report on the use of low-temperature epitaxy of Ca and Mg under oxidizing conditions to yield mixed thin film oxides. Low-energy electron diffraction showed that the mixed oxides grew as single crystals over a wide range of compositions. We compare the water adsorption characteristics of the mixed oxide to that of CaO and MgO and find that the mixed oxide behavior is not a linear combination of the two pure oxide systems. The thermal stability of the mixed oxide film was checked by Anger electron ratios for an oxide-oxide "sandwich" after various "flash" anneals. This and water adsorption results suggest intimate atomic-scale mixing exists until flashed above 1100 K.