Near-infrared absorption of the OH stretching overtone transition of water has been measured at temperatures and pressures in the ranges of 373-673 K and 20-400 bar, respectively. The absorption profile at 673 K and 400 bar retains a mark of rotational structure, indicating that an appreciable proportion of water molecules can rotate quite freely. The molar absorption intensity decreases linearly with increasing pressure in the low-pressure region. Enthalpy for dimerization has been estimated to be 15+/-3 kJ/mol from the temperature dependence of the slopes. Plots of the molar absorption intensity against molar concentration are observed to be located on a single curve irrespective of the temperature. This fact indicates that the ratio of hydrogen-bond formation is largely dependent on the molar density only. A good correlation between the molar absorption intensity and the first moments of the band has been found out; this will be useful in the study of aqueous mixtures. (C) 2003 American Institute of Physics.