X-ray diffraction is applied to determine the degree of order in partially ordered epitaxial layers of ZnSnP2 grown on GaAs. The Bragg-Williams order parameter, used as a scaling coefficient for the structure factor of superstructure reflections, is extracted from the comparison of measured and calculated relative intensities of a set of carefully chosen reflections. The calculated diffraction patterns are obtained from the dynamical theory of x-ray diffraction. The effect of antiphase domains on the width of superstructure reflections is discussed. Order parameters up to 30% were measured.