Summary: The usual method for determining the gas in place (GIP) in a reservoir uses a balance equation based on laboratory measurements of the raw gas volumetric factor as a function of pressure on a water-free basis. In the reservoir, however, the gas is saturated with water. Volumetric measurements at high temperatures and pressures have been performed on a North Sea fluid at 374°F, both in water-free conditions and in the presence of an excess water. The water content of the gas at 650 bar has been determined to be 3.5 mol% (3.6 mole of water/100 mole of reservoir fluid) water-saturated reservoir fluid. A corresponding volume increase of 3.2 % has been detected. These experimental measurements have been correlated with a Tait-type equation. The S0reide and Whitson1 model has been used to calculate the water content of the gas. A modified balance equation is proposed for prediction of the GIP.