Adsorption of diazine dye safranine O (SO) in the presence of Cs+ and Sr2+ ions was investigated onto natural and synthetic zeolites in order to predict competition of cationic organic species with their radionuclides, which are the main fission products released into the environment. Adsorption of SO was measured up to the 40th day and the surface-diffusion coefficients (D-s) were estimated by applying Nernst-Planck approximation based on a homogeneous-surface-diffusion model. The values of D-s were 10times higher on natural zeolite than those of synthesized zeolite from fly ash (FA) under hydrothermal conditions. Similarly, distribution coefficients (K-D) were considerably higher on the clinoptilolite-type natural zeolite. The zeolitized product of FA is mainly composed of analcime and sodalite. SO adsorption on natural zeolite was not influenced by Cs+ and Sr2+ ions, but it decreased at high concentrations on synthetic zeolite. The higher influence of the Sr2+ ions on SO+ adsorption showed that they compete with each other for the same adsorption sites. These results suggested that natural zeolite cannot be used for remediation of wastewater polluted with Cs and Sr radionuclides in the presence of organic cations, whilst FA zeolite has a potential for Sr removal.