It is the purpose of this paper to present an infrared absorption spectroscopic study of the state of aggregation of water over a wide range of temperature (25–380°C) and pressure (1–250 bar) along the liquid-gas coexistence curve, and in the supercritical domain. The evolution of the spectral profiles asociated with the internal vibrational modes and with the librational motion has been investigated. In supercritical water, at T=380°C, low pressures (densities) in the range 25–50 bar (0.01–0.05 g.cm−3), only monomeric water is detected. Progressive increase of the pressure (density), from 50 to 250 bar (from 0.05 to 0.4 g.cm−3), shows clearly the appearance of water dimers and trimers. Finally, upon decreasing the temperature (250-25 °C) along the liquid-gas coexistence curve, one observes a continuous evolution of the shape of the infrared spectrum characteristic of the presence of oligomers of increasing size and for temperatures lower than 200°C, the progressive appearance of the hydrogen bond network.
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