A lattice statistical mechanical method has recently been developed and used to predict structural and thermodynamic properties of fluid phases of surfactant monolayer films of uniform chain length. This mean-field approach is extended here to examine films of mixed chain length. As in the earlier work, a fluid-fluid (LC-LE) phase transition is predicted to occur at high surface densities. This transition is found to exhibit non-ideal mixing behavior, ranging from positive azeotropy to phase separation (in the LC phase) as the difference in chain lengths increases. In contrast, the phase diagram in the liquid-gas (LEG) transition region is predicted to exhibit almost ideal behavior, with a nearly linear bubble-point curve.