This paper is the background for a new international formulation for the ion product of water substance (May 1980) issued by the International Association for the Properties of Steam. The ion product of water (Kw) is represented by an equation, based on density and two quadratic functions of reciprocal absolute temperature, for use from 0 to 1000 °C and 1 to 10,000 bars pressure. The equation is believed to describe within ±0.01 units of logKw*(whereKw*equalsKw/(mol kg−1)2) many of the measurements at saturated vapor pressure up to 200 °C, and to within ±0.02 units up to the critical temperature (374 °C). It also describes within the experimental uncertainty the several sets of measurements at high pressures and should provide values within ±0.05 and 0.30 units at low and high temperatures, respectively.