Many Zr-containing phases of the sodium zirconium phosphate (NZP) type structure have been evaluated as host materials for the immobilization of radionuclides. Phosphates crystallizing in eulytine type structure are identified as promising crystalline hosts matrices for the immobilization of rare earth cations and are tested for the first time. The soxhlet tests performed on these two kinds of phosphates with different crystal structure show that the Ln3+ in eulytine and Zr4+ in the NZP phases show very low leach rates of the order of 10-5 to 10-6gm2 day even in powdered samples. The leach rates attain levelling after 4-5 days. It is found that the leaching process is controlled by both diffusion and dissolution, The leaching process of the individual species is found to depend on the charge density of the respective ions present in the matrix.