The new technique of diffusive gradients in thin films (DGT) has been used for the first time with a general cation exchange resin (AG50W-X8) as the binding agent. Its use for the measurement of Cs and Sr has been systematically investigated. Individual experiments showed that resin embedded in polyacrylamide gel efficiently removed Cs and Sr from solution. Cs and Sr could be reproducibly eluted with nitric acid if sufficient volume was used. The dependence of the DGT response to exposure time, gel layer thickness and temperature could be theoretically predicted for a wide range of pH (4–9) and ionic strength (1 μmol l−1–1 mmol l−1). The major difference compared to the use of a highly selective resin, such as Chelex, for trace metals, is that the resin becomes saturated due to the continuous uptake of the major cations present in solution. Because of this capacity limitation, the theoretical response in soft water was only obtained for exposure times up to 20 h. The use of DGT with general purpose resins to measure ions in natural waters is likely to be restricted to soft water. In situ measurement of labile species of Cs (0.24 nmol l−1) and Sr (0.19 μmol l−1) in soft water showed no evidence for the presence of stable complexes or colloidal forms.