Carbon isotope analyses of calcite veins, organic carbon. CO2 and CH4 from 96 rock and 46 gas samples show that metamorphic calcite veins and disseminated, organically-deriv ed carbon from Franciscan Complex and Great Valley Sequence rocks have provided a primary carbon source for geothermal fluids during past and present hydrothermal activity across The Geysers-Clear Lake region. The stable isotope compositions of calcite veins vary widely on a regional scale, but overall they document the presence of C-13-poor fluids in early subduction-related vein-precipitating events. delta C-13 values of calcite veins from the SB-15-D corehole within The Geysers steam held indicate that carbon-bearing fluids in the recent geothermal system have caused the original diverse delta C-13 values of the veins to be reset across The Geysers-Clear Lake region the carbon isotope composition of CO gas associated with individual geothermal reservoirs shows a general increasing trend in delta C-13 values from west to east. In contrast, delta C-13 values of CH4 do not exhibit any spatial trends. The results from this study indicate that regional variations in delta C-13-CO2 values result from differences in the underlying lithologies. Regional CO2 contains significant amounts of carbon related to degradation of organic carbon and dissolution of calcite veins and is not related to equilibrium reactions involving CH4. CO from degassing of underlying magma chambers is not recognizable in this region. Published by Elsevier Science Ltd on behalf of CNR.