Using data from selected geothermal fields in Iceland, several gas geothermometers are applied to calculate reservoir temperatures. Results for well fluids are compared to estimated inflow temperatures. The effects of gases from other sources, e.g. magmatic fluid entering reservoirs, condensation/boiling, loss of components and mixing of different fluids during upflow, are discussed. Those geothermometers that give results reasonably close to the inflow temperatures, and are not constrained by the need to know thermodynamic parameters, are used to estimate subsurface temperatures from fumarole steam composition. It is suggested that geothermometers based on H-2 and H2S concentrations are relatively effective. The CO2, H2S and H-2 geothermometers are also evaluated by thermodynamic simulation using plausible mineral buffers. The CO2 geothermometer generally gives slightly low temperatures for values in the range 100-200 degrees C. The calibration of the existing H2S and H-2 geothermometers needs improvement when reservoir temperatures are higher than 220 degrees C. The mineral buffer controlling H2S in reservoirs seems to be pyrite + magnetite f epidote + prehnite changing into pyrite + pyrrhotite + epidote + prehnite with increasing temperature.