Thin films of different molybdenum carbides (delta-MoC1-x, gamma'-MoC1-x and Mo2C) have been deposited from a gas mixture of MoCl5/H-2/C2H4 at 800 degrees C by CVD. The H-2 content in the vapour has a strong influence on the phase composition and microstructure. Typically, high H-2 contents lead to the formation of nanocrystalline delta-MoC1-x films while coarse-grained gamma'-MoC1-x, is formed with an H-2-free gas mixture. This phase has previously only been synthesized by carburization of Mo in a CO atmosphere and it has therefore been considered as an oxycarbide phase stabilized by the presence of oxygen in the lattice. Our results, however, show that gamma'-MoC1-x films containing only trace amounts of oxygen can be deposited by CVD. Stability calculations using a FP-LMTO method confirmed that the gamma'-MoC1-x phase is stabilized by oxygen but that the difference in energy between e.g. delta-MoC0.75 and oxygen-free gamma'-MoC0.75 is Small enough to allow the synthesis of the latter phase in the absence of kinetic constraints. Annealing experiments of metastable delta-MoC1-x and gamma'-MoC1-x films showed two different reaction products suggesting that kinetic effects play an important role in the decomposition of these phases.