The activity and selectivity of various MnOx catalysts for the selective reduction of nitrobenzene to nitrosobenzene have been studied, measuring the rate in a continuous flow reactor. Characterisation of the catalysts has been performed with XRD, IR, UV/VIS, EM, and TPR. It appeared that the steady-state catalyst is in all cases the spinel Mn3O4 in a slightly reduced form. Without the addition of an external reductant to the feed, nitrobenzene was not only reduced but also oxidised, which created oxygen vacancies in the catalyst surface. Addition of an external reductant did not cause a change in the amount of nitrosobenzene formed, but the concentration of side products, like aniline and dimerisation products, increased. It is suggested that the selective reduction of nitrobenzene with the catalyst Mn3O4 runs by the Mars and Van Krevelen mechanism.