The rutile form of titanium dioxide (TiO2) and granules of high density polyethylene (PEHD) and low density polyethylene (PELD) were used to prepare mortar matrices for immobilization of radioactive waste materials containing ‘137Cs. PELD,PEHD and TiO2 were added to mortar matrix preparations with the objective of improving physico-chemical characteristics of the radwaste-mortar matrix mixtures, in particular the leach-rate of the immobilized radionuclide. The diameters of the PELD and PEHD used varied from 0.2 to 2.0 mm. One type of PELD and two types of PEHD were used to replace 50 weight percent of stone granules, average diameter of 2 mm, normally used in the matrix, in order to decrease the porosity and density of the mortar matrix and to avoid segregation of the stone particles at the bottom of the immobilized radioactive waste cylindrical form. TiO2 was also added to the mortar formulation, replacing 5 and 8 weight percent of the total cement weight, for each PEHD and PELD formulation. Cured samples were investigated under temperature stress conditions, where the temperature extremes were: Tmill = -20°C,Tmax= +70°C. Samples were periodically immersed in distilled water at the ambient room temperature, after each freezing and heating treatment. Results of accelerated leaching experiments for these samples and samples prepared exclusively with polyethylenes replacing 100 percents of the stone granules and TiO2, treated in nonaccelerated leaching experiments, were compared. Even using an accelerated ageing leach test that overestimates 137Cs leach rates, it can be deduced, that radionuclide leach rates from the radioactive waste mortar mixture forms were improved. Leach rates decreased from 5 percent, for the material prepared with stone aggregate, to 3.1 to 4.0 percent, for the materials prepared solely with PEHD, PELD or TiO2, and to about 3 percents for all six types of the TiO2-PEHD and TiO2-PELD mixtures tested.