A series of gamma-Al2O3 supported Mo nitrides were prepared by the temperature programmed reaction of supported molybdates with NH3. Several loadings, heating rates, and space velocities were employed in an attempt to vary the properties of the nitride. The structure and composition of the supported nitrides depended on the structure of the precursor oxide and the conditions employed in nitriding the oxide. The ease with which the oxide was nitrided improved with increasing Mo loading. Nitride domains in the low-loaded materials (4 and 8 wt% Mo) were highly dispersed and X-ray amorphous while the higher-loaded catalysts (16 wt% Mo) consisted Of gamma-Mo2N crystallites. The response of the high-loaded materials to the various nitriding conditions was similar to that observed for unsupported gamma-Mo2N. The supported Mo nitrides were active for the hydrodenitrogenation of pyridine. In fact, their activities were superior to that of a commercial sulfided Ni-Mo hydrotreating catalyst and comparable to those of the unsupported Mo nitrides. The catalytic properties of the supported Mo nitrides depended on the size and composition of the Mo nitride domains. We believe that the most active sites were located at the perimeters of two-dimensional, raft-like domains. Regions near the perimeter also appeared to be nitrogen deficient. Lower activity sites were associated with the gamma-Mo2N crystallite surfaces.