The oxide and sulfide forms of commercially available CoMo γ- Al 2O 3 have been studied with Extended X-ray Absorption Fine Structure (EXAFS). Structural information was obtained for sulfiding temperatures from 25 to 600 °C. The oxygen coordination is somewhat uncertain for the oxide form of the catalyst, but values of 3 to 4 are indicated. Neither MoO 3 or α-CoMoO 4 are present. Very mild sulfiding conditions result in formation of MoS coordination with no measurable MoS 2, but at increasing sulfiding temperatures, MoS 2 crystallites are formed, their size increasing at the higher temperatures. Crystallite sizes ca. 15 Å are evident between 300 and 400 °C. S Mo ratios of 2 are calculated from EXAFS, considerably lower than implied by total sulfur uptake, with a value of 3, and MoS coordination numbers no greater than 4 are observed.