The wettability in gas-liquid-rock systems, including geothermal (steam-water-rock) systems, is central to defining the relative permeability and capillary pressure, which govern the forecasts of future production and reinjection performance. Since the frequently used Amott and USBM approaches are appropriate only for specific liquid-liquid-rock systems, we developed a model to evaluate the wettability in gas-liquid-rock systems. The method can be applied to determine the wettability at specific wetting-phase saturation if the capillary pressure and relative permeability at this fluid saturation are known. The proposed model can also be used in liquid-liquid-rock systems. The validity of the technique was tested qualitatively and quantitatively using experimental data from different rock-fluid systems. The calculated wettability indices in gas-liquid-rock systems were higher than in liquid-liquid-rock systems, as expected. Results also show that the wettability index calculated using the new method may or may not be independent of fluid saturation in the cases Studied. In addition, the wettability index in drainage was seen to be greater than that in imbibition, a wettability difference that is particularly important in steam-water-rock (geothermal) systems because of the impact on reinjection. (C) 2008 Elsevier Ltd. All rights reserved.