The influence of the support on the performance of Mo nitrides has been investigated in ammonia synthesis and decomposition. A series of Mo–N catalysts supported on different materials, namely SiO2 (commercial, SBA-15), Al2O3, and CeO2, were prepared. The results indicated that, despite the high dispersion of Mo species in all catalysts, large disparities in the activity for ammonia synthesis exist. Initial rates of ∼1208, ∼481, and ∼372 µmol gactive phase−1 h−1 are obtained over 10-Mo–N/SBA-15, 10-Mo–N/SiO2, and 10-Mo–N/Al2O3 respectively. However, no catalytic activity was registered when Mo species were supported on CeO2. Furthermore, 10-Mo–N/Al2O3 deactivated after few hours of reaction. The surface composition was studied by means of XPS to probe the origin of the catalytic activity differences, and the results indicated that a range of various oxidation states of Mo was detected depending on the support. The difference in catalytic behavior could not be solely explained by the differences in Mo–N species concentrations. In situ EPR analysis exhibited that the mechanism of MoO3 nitridation could differ depending on the support, leading to the formation of different Mo–N species. The effect of support was, however, not as severe in ammonia decomposition as it was the case of ammonia synthesis.