The liquid-phase oxidation of benzene was carried out in aqueous acetic acid solvent over V-substituted heteropolyacids (V-HPAs) using molecular O-2 and ascorbic acid as the oxidant and reducing reagent, respectively. Phenol was exclusively obtained as the oxygenation product. The elimination of the V species from the V-HPA (Keggin structure) catalyst into the reaction solvent during the benzene oxidation reaction was largely inhibited by ion-exchanging the proton of the V-HPA catalyst with Cs+. The main active species were assumed to be the V species anchored in the HPA. The influences of the reaction temperature, the concentration of acetic acid in the aqueous solvent, and the reaction pressure on the yield of phenol were investigated to obtain the optimal reactions condition for phenol formation. The reuse of the V-HPA catalyst caused gradual deactivation for phenol formation, despite the retention of the structure of the V-HPA catalyst. The deactivation was suggested to be due to the reduction of the V species in the V-HPA catalyst on the basis of the diffuse reflection spectra of the used catalysts.