A water-benzene mixture has been studied by infrared spectroscopy at temperatures and pressures in the 473-648 K and 100-350 bar ranges. The OH stretching band intensity has been obtained as a measure of water concentration in the mixture. In the temperature range of 473-523 K, the intensity is almost independent of pressure. On the contrary, at higher temperatures, the absorption intensities exhibit remarkable pressure dependence. Moreover, the intensities increase with increasing temperature towards a certain temperature and then steeply decrease at higher temperatures. It has been found that the turning points of the intensities are located on an extended line of the liquid-liquid-gas three-phase equilibrium curve of the water-benzene mixture and near the critical curve between the two-phase and one-phase regions. This fact suggests that density of the mixture changes steeply across those lines. We have also observed coalescence of two bands, which are assigned to hydrogen-bonded and hydrogen-bond-free OH, respectively, at the higher temperatures, and discussed rotational motion of a water molecule in a hydrogen-bonded cluster.