The concentrations of CO2, H2S, and H2 in geothermal reservoir fluids in Iceland are fixed by equilibria with alteration minerals. CH4 concentrations are typically very low, and gas-gas equilibria involving CO2, H2, and CH4 are generally not closely approached. In some geothermal systems the N2 is atmospheric only, but a magmatic N2 contribution is significant in others. N2/Ar ratios lie mostly between that of the atmosphere (85) and the N2/Ar ratio in cold water in contact with the atmosphere (37). Fluids with a strong magmatic N2 contribution have N2/Ar ratios of 40--60. During several volcanic eruptions in Iceland, magmatic gases have emerged at the surface beyond the eruptive vents and mixed with surrounding groundwaters. CO and H2 can be detected closest to the vent, but only CO2 is detected at greater distances. During the present volcanic episode at Krafla, fugitive constituents have been transported into the geothermal reservoir from the underlying magma chamber. Probably most of the carbon and sulphur in altered rocks of high-temperature geothermal systems have been derived from degassing magma bodies.