Emission of methane (CH and CO2. In situ mole fractions of CH4 and CO2 were measured in the subglacial air at a subglacial river outlet where emissions of CH4 and CO2 had been identified. Water samples were analysed for dissolved CH4 and CO2 concentrations. CH4 and CO2 in gas and water samples were analyzed for their isotopic composition of 13C and 2H. Close correlation between gaseous and dissolved CH4 and CO2, respectively, show degassing of CH4 and CO2 from the subglacial meltwater. The diurnal variability of in situ mole fractions of CH4 and CO2 in subglacial air was related to meltwater runoff. Maximum in situ mole fractions decreased after the peak of the melt season, but estimated net emissions increased because the size of the subglacial river outlet increased. The isotopic signature of CH4 in the subglacial air, estimated with a Keeling plot, indicated that subglacial CH4 likely originated from acetoclastic methanogenesis. Isotopic signatures of gaseous CO2 indicate that both microbial oxidation of CH4 in the subglacial system and remineralization of carbon in subglacial sediments contribute to subglacial CO2.