The u.v., visible, and i.r. spectra of the following compounds containing cobalt, molybdenum, and aluminium in tetrahedral and octahedral co-ordination in oxide lattices, and the magnetic susceptibilities over the temperature range 80–380°K of the first four compounds are reported: hydrated cobalt molybdate, calcined cobalt molybdate (CoMoO4), cobalt(II) molybdenum(IV) oxide (Co2Mo3O8), cobalt aluminate, cobalt(II) oxide, cobalt(II,III) oxide (Co3O4), sodium molybdate, ammonium paramolybdate, molybdenum trioxide, and γ-alumina. The spectroscopic and magnetic properties of a fresh (i.e., unsulphided) cobalt–molybdenum–alumina hydrodesulphurisation catalyst are reported and compared with the properties of the above compounds in order to determine the chemical and structural environment of cobalt and molybdenum in the catalyst. Cobalt is present as cobalt(II), approximately 50% in octahedral co-ordination and 50% in tetrahedral co-ordination by oxide. There is very little magnetic interaction between the cobalt ions which are well dispersed in the catalyst. Molybdenum is present as molybdenum(VI) in tetrahedral co-ordination by oxide. There is no evidence for the presence in the catalyst of Co3O4 or any well defined compound of cobalt, molybdenum, or aluminium. The relevance of the results to the preparation of the catalyst is discussed.