The reactions of cyclohexene (Ch) and tetralin (T) in the pure liquid state and in their solutions in chlorobenzene were found to exhibit a strong dependence on the concentration of the catalyst VO(acac)(2): a double maximum curve was observed in consequence of the presence of the catalyst in various forms as a result of its tendency to undergo aggregation. The monomer form of the catalyst below 10(-4) M proved more active than the dimer (or oligomer) at higher concentrations. However, only a single maximum was observed when toluene was applied as solvent in place of chlorobenzene. In a closed reactor system, the oxidation of Ch produced Ch-OOH, Ch-O (epoxide) and Ch-ol, but Ch-one was never found. In the oxidation of T, T-OOH, T-ol and T-one were obtained. In some cases, the sum of the oxidation products was greater than might be expected from the measured O-2 uptake, because of the disproportionation of T-OOH into O-2 and T-ol. It was observed that the catalyst is able to interact with and is thereby activated by T. Consequently, the oxidation of T can start in the absence of a hydroperoxide initiator; this was not the case, however, in the oxidation of Ch. The ion-pair VO2+-aerosol-OT also exerted a catalytic effect, but with lower activity, and a double maximum was not produced on increase of the ion-pair concentration. (C) 2002 Elsevier Science B.V. All rights reserved.