On the way to perfectly lattice-matched Cu(In,Ga)S-2, we present the first CuGaS2 films epitaxially grown on Si substrates and compare their structural properties to epitaxial CuInS2. on the same type of substrate. Both of these chalcopyrites have been grown on sulfur-terminated Si(111) and showed a six-fold surface symmetry in low energy electron diffraction. Composition was controlled in and ex-situ using Auger electron spectroscopy and Rutherford backscattering, respectively. A detailed film analysis by means of X-ray diffraction methods yielded the lattice parameters of the epitaxial layers, wider rocking curves of the gallium compound as compared to the indium compound, and showed the suppression of twin formation in Ga-rich CuGaS2,. The results indicate that a perfect lattice match between Cu(In(lambda-x)Galambda)S-2 and Si will be attainable with x approximate to 0.5.