Study of137Cs in rivers implies knowledge of its adsorption behaviour towards particulate matter and sediments and the possibility of low-level counting of water and solids. As the adsorption isotherms and kinetics will be discussed separately, this text focuses on the low level counting of137Cs. The use of flat-ended Ge(Li) detectors is compared to that of an anticoincidence array after preconcentration of the cesium ions into a 0.5 ml solid aliquot. The lower limits of determination at a relative standard deviation of 10% are 10 Bq · kg–1 for as received aliquots on a flat Ge(Li) detector and 10 mBq in case of preconcentration from water and counting in the anticoincidence facility. The eventual improvement of the accuracy by applying moving average and smoothing procedures are considered.