Leaching characteristics of some radionuclides that are commonly encountered in radioactive waste streams from immobilized waste matrices in different cement-clay grouts have been assessed to investigate the influence of the clay additives on the leaching behavior of the solid waste matrices. The International Atomic Energy's Agency (IAEA) standard leach method has been employed to study the leach pattern of Cs-137, Co-60, and Eu-152,Eu-154 radionuclides immobilized in ordinary Portland cement (OPC), OPC-bentonite, and OPC-red clay grouts. The examination of the leaching data revealed that adding clays to OPC reduces the leach pattern as OPC-bentonite < OPC-red clay < OPC for all studied radionuclides. The controlling leaching mechanism has been found to be diffusion and the mean apparent diffusion coefficients and leachability indices have been calculated for the studied waste matrices. Simplified analytical models have been derived to predict the cumulative leach fraction (CLF) of radionuclides over the studied experimental period. These simplified research models could be used as a screening tool to assess the performance of the waste matrix under repository conditions. (c) 2006 Elsevier B.V. All rights reserved.