Dissolved inorganic carbon (DIC) is the main acid buffer in forested lake watersheds in Canada. We usedcarbon isotopes (13C, 14C) to evaluate the production and cycling of DIC in an acid-sensitive lake watershed of thePrecambrian Shield. Soil C02, groundwater and stream DIC were characterized chemically and isotopically. Soil CO2 concentration profiles reflect both changes in production and in losses due to diffusion. S13C soil CO2 profiles (b13C values of -23%o in summer, slightly enriched during the fall and -25%o during the winter) are a reflection of the isotopic composition of the sources and changes in isotopic fractionation due to diffusion. Carbon isotopic composition (13C, 14C) of the groundwater and stream DIC clearly indicate that weathering of silicates by soil CO2 is the main source of DIC in these watersheds. 14C data show that, in addition to recent groundwater, an older groundwater component with depleted 14C activity is also present in the bedrock. The carbon isotope pattern in the groundwater also implies that, besides the main springtime recharge events, contributions to the groundwater may also occur during late winter/early spring.