MoCx films were grown by d.c. reactive sputtering in an Ar-CH4 atmosphere onto MgO (001) single-crystal surfaces. The crystallographic structure of the films was revealed by reflection and transmission high-energy electron diffraction. The microstructure was studied by transmission electron microscopy and the composition of the films was determined by Auger electron spectroscopy. Under the used processing conditions, the electron diffraction shows that only MoC with the B-1 structure (NaCl-type) was formed. The carbon content of the films was found to increase with increasing CH4 partial pressure and the lattice parameter of the B-1 phase was also found to vary in the same direction as a consequence of an increase of incorporated carbon in the B-1 lattice. The determined values for lattice parameters and carbon contents, in the carbon-richer side, were somewhat greater than the highest ones reported for the rapidly quenched B-1 phase. This fact was related to an extension of the homogeneity range of this phase. The results may be attributed to lower deposition rates which seem to play a prominent role. In agreement with previous studies performed on analogous reactively sputtered M-C systems, the grain size was found to decrease when the concentration of reactive gas is increased. This behavior may be interpreted in terms of carbon location at the grain boundaries as well as in terms of a reduction of the mean migration paths of adatoms caused by an increase of the substrate impinging Bur resulting from more CHn radical formation.