The present investigation deals with cobalt molybdate and the cobalt oxide molybdenum oxide system. It is a prelude to an investigation of the CoO-MoO3-Al2O3 hydrodesulfurization catalyst. DTA and X-ray spectroscopy show that compound formation only occurs at the composition CoMoO4. Two modifications of CoMoO4 were prepared. DTA shows the existence of two transition temperatures of the modifications into each other. Infrared spectra indicate that the molybdenum is octahedrally surrounded by oxygen in both modifications. The surrounding of the cobalt remains undetermined from reflection spectra, but from measurements of the magnetic susceptibility it follows that the cobalt is octahedrally surrounded in both modifications.