The different modes of interaction of La3+ and Ce4+ as simulated radioactive lanthanides found in the backend of the nuclear fuel cycle with polyethylene and polypropylene macromolecules were considered. The various compositions were characterized before and after irradiation at different gamma ray doses. The mechanical properties including stress-strain parameters were also tested. The morphological and mechanical behaviors of the various composites were different from those of the corresponding pure polymers and affected by the chemistry of the interacting elements. Besides, the leaching characteristics were carried out using the short- and long-term procedure; the leaching rates were evaluated as an index for immobilization of these lanthanides. Two semiempirical models were applied to find out the dominant leaching mechanism of La3+ and Ce4+. Dissolution associated with diffusion was the dominant leaching path and the elemental leaching characteristics depended upon its solubility in water. Diffusion coefficients and dissolution rate constants, the main parameters in the long-term prediction model, were obtained using short- and long-term experimental data. The model was found to be useful in predicting the long-term behavior of such elements to assess the stability of the used waste forms. (c) 2006 Wiley Periodicals, Inc. I Appl Polym Sci 103:2141-2151, 2007