Infrared spectra of water-benzene mixtures have been observed at temperatures and pressures in the 473-648 K and 100-350 bar ranges. The OH stretching band intensities of HDO in the benzene-rich phase in the two-phase region and in the one-phase region were obtained as a function of temperature and pressure. The band intensity, as a measure of water concentration, increases by about three times as the temperature rises from 473 to 523 K, while it is almost independent of pressure in the 100-350 bar range at these temperatures. At higher temperatures, on the contrary, the absorption intensities exhibit remarkable pressure dependence. They increase by an order of magnitude as the pressure increases from 100 to 350 bar. These temperature-pressure dependent changes of water concentration can be properly understood with a phase diagram of the water-benzene mixture. The absolute concentration of water in the benzene-rich phase and in the homogeneous one phase has been estimated from the absorption intensities by using a simple empirical relationship. The resulting values are consistent with literature data, which were obtained by composition analyses of the mixtures. The effect of temperature and pressure on the water solubility in the benzene-rich phase has been argued in terms of energies of solvation and cavity creation for a solute molecule. Coalescence of the hydrogen-bonded and hydrogen-bond-free OH stretching bands has been observed at higher temperatures and discussed in terms of rotational motion of water molecules in a hydrogen-bonded cluster.