The concentrations of some fission products in rain water falling on the catchment areas of three Thames tributaries are compared with the concentrations in river water over monthly periods. The fission products selected were SrBQ, SrQo, Cs13' and GelP4.A comparison of the concentrations in corresponding samples of rain and river water showed that natural decontamination factors were smallest for SrQo and greatest forThe Srso concentrations in river water appear to depend to some extent on the permeability of the subsoil in the catchment area, which suggests that surface run-off is an important source of contamination. The amount of Sr9o leaving each catchment area in the river is compared with the amount deposited. Normally less than 1 per cent leaves in the river, but in periods of heavy rain and high flow rates this may be considerably increased due to desorption from the soil and enhanced run-off.The effect on the Sr90 concentration in river water of (a) increased testing of nuclear weapons and (b) a reactor accident are discussed briefly.