The genetic relationship between carbon-bearing species (CO, CO 2 , CH 4 , C 2 H 6 , C 3 H 8 , C 2 H 4 and C 3 H 6 ) was investigated in volcanic-hydrothermal gases emitted from Nisyros (Greece), Vesuvio, La Solfatara (Campi Flegrei) and Pantelleria (all Italy).Apparent carbon isotopic temperatures of the CH 4 –CO 2 system are ~360°C at Nisyros, 420–460°C at Vesuvio, ~450°C at La Solfatara and ~540°C at Pantelleria. These temperatures are confirmed by measured propene/propane and H 2 /H 2 O concentration ratios. CH 4 and CO 2 equilibrate in the single liquid phase prior to the onset of boiling, whereas propene and propane attain equilibrium in the saturated water vapor phase. Boiling in these high-enthalpy hydrothermal systems might occur isothermally. Once vapor has been extracted from the parental liquid, CO/CO 2 responds most sensitively to the temperature gradient encountered by the ascending gases.Our results imply that the CH 4 –CO 2 isotopic geothermometer can provide reliable information about temperatures of deep hydrothermal liquids associated with volcanism. Propene/propane and H 2 /H 2 O concentration ratios should be measured along with the carbon isotopic composition of CO 2 and CH 4 to provide independent constraints on the geological significance of the apparent carbon isotopic temperatures.