Volume behavior of water-benzene mixtures at temperatures and pressures in the 473-623 K and 100-350 bar ranges, respectively, has been studied by infrared in situ measurements. The densities of the benzene-rich phase were estimated from the spectroscopically determined concentrations of water and benzene and compared with the average densities before mixing, which were calculated using literature densities of neat water and neat benzene at the same temperature and pressure. Anomalously large volume change for mixing has been found in the vicinity of the critical region of the water-benzene mixtures.