Stable isotope characterization of porewater, and dissolved species, in mudrocks and argillaceous rocks is notoriously difficult. Techniques based on physical or chemical extraction of porewater can generate significant analytical artefacts. The authors report a novel, simple approach to determine the δ18O of porewater and δ13C of dissolved C in argillites. The method uses core samples placed in specifically-designed outgassing cells, sealed shortly after drilling and stored in well-controlled conditions. After 1-2 months, CO2 naturally outgassed by argillite porewater is collected, purified and analyzed for C and O isotopes. Porewater δ18O and dissolved C δ13C are calculated from CO2 isotope data using appropriate fractionation factors. This methodology was successfully applied to the Callovo-Oxfordian argillites from Bure (eastern Paris Basin, France) and the Opalinus Clay formation from Mont Terri (Switzerland). In both clay formations, results indicate that porewater is meteoric and dissolved C is of marine origin. The main advantage of the approach is that it does not induce any major physical or chemical disturbance to the clay-water system investigated. Further testing on argillaceous rocks of variable composition and organic content is needed to assess extent of applicability.