As a consequence of an accidental fusion of a Cs-137 source in a steel production factory, 40 m(3) of radioactive liquids were generated in the plant decontamination process. This radioactive liquid waste was treated by a reverse osmosis plant, managing to declassify more than 36 m(3). The remaining 4 m(3) were treated by evaporation reducing the final volume to less than 1 m(3). Prior to the waste treatment at the steel factory, preliminary tests were done in order to determine reverse osmosis membrane performance with Cs-137 effluents. The influence of gamma and electron irradiation on structure and transport properties of reverse osmosis membranes was studied; and reverse osmosis performance with a Cs-137-contaminated liquid obtained from contaminated ashes was assessed. The aim of this paper is to describe and show the results of these preliminary tests.