The adsorption of nitrobenzene on unsupported and SiO2-supported Co3O4 and a range of adsorbed reaction products of nitrobenzene are studied by infrared spectroscopy. Products observed after adsorption of nitrobenzene on Co3O4 are aniline, azoxybenzene, and cyclohexylamine (reduction products) at lower temperatures and carbonates and cyanates (oxidation products) at higher temperatures. One of the supported cobalt oxides appears to be very different from the pure Co3O4, probably due to cobalt silicate formation during its synthesis. With all samples, the surface reactivity and adsorption of gases are mostly related to hydroxyl groups, but Lewis acid sites ace also present on the surface.