Langmuir-Blodgett films of mixed behenic and stearic acid salts of cadmium were deposited and investigated by small-angle X-ray scattering. The ratio of compounds was varied from 1:4 to 4:1. It was found that for 1:1 ratio the lattice spacing was equal to 54.0 Å, which corresponds to double chain length of arachidic acid or to a sum of one chain length of stearic and behenic acid salts. In this case a single completely mixed phase was obtained. For the other ratio X-ray patterns contained two systems of reflections, each of them corresponding to one of the pure fatty acid salts (49.0 Å and 58.5 Å respectively), implying the coexistence of two phases.