The activities for the vapour-phase oxidation of toluene of various molybdenum-containing catalysts have been measured in a flow microreactor operating at 1 atm pressure. The catalysts comprised (a) unsupported crystalline MoO3, (b) Mo oxide monolayers on Al2O3, CeO2, TiO2, and ZrO2 supports, and (c) Al-, Ce-, and Zr-molybdate salts. The rates of oxidation of toluene per unit surface area on the monolayer catalysts are higher than those on the corresponding salts, but the selectivities for the partial oxidation products show the opposite trend. The catalytic behaviour of unsupported MoO3 resembles more closely that of the salts than that of the monolayer catalysts. The kinetic data for the oxidation of toluene on all catalysts can be interpreted on the basis of a reduction-oxidation mechanism. The energies of activation for the reduction and oxidation steps are correlated with the ionic potentials of the cations in the supports. Poisoning by pyridine of MoOx/TiO2 has a profound effect on the activity and selectivity of the monolayer catalyst and gives some information about the nature and concentration of the active sites.