A method for the analysis of the physico-chemical forms of radiostrontium in waters is described and applied to the simulated lake and groundwaters to which were later added clay minerals and humus substances. The simulated water samples were fractionated using filtration and ultrafiltration: particulates, colloids and dissolved species. The dissolved species were further separated into three fractions using ion exchange: cations, anions and non-ionic species. The physico-chemical forms of radiostrontium do not appreciable change when humus substances or clay mineral or both are added to the water samples. Only about 1–6% of added radiostrontium was found in particulate and colloid fractions. The greatest amount of85Sr was always found in the dissolved fraction, cations (>87%). Radiostrontium was never found in the dissolved fraction, anions or non-ionic species.