The performance of pellets of unsupported and silica-supported Co3O4 in the ammonia oxidation was investigated as a function of the particle size to investigate the utilization of the catalytically active phase in these materials. The obtained activity in terms of ammonia conversion over the silica-supported Co3O4 is higher compared to the conversion over the unsupported Co3O4, despite a lower cobalt oxide loading and more severe diffusional limitations. The effectiveness factor for the silica-supported catalyst is slightly lower than the effectiveness factor for the unsupported catalyst in the form of pellets of similar size. However, the effective utilization of cobalt within the catalyst is higher for the silica-supported catalyst, mainly due to the higher dispersion of the catalytically active phase.