The aim of the research presented in this thesis was the study of the selective sorption of radioactive cesium and strontium in Pillared Layered Clays (PILCs). Two reference samples, coded ATFflNA and AZA, were initially used in experiments both in vitro and in vivo. An improved pillared clay, coded FRAZA, was constructed, characterised by several methods (XRD, XRF, PIGE, N 2 adsorption-desorption, radiotracers' exchange isotherms) and employed in exchange experiments. The reference samples and the material FRAZA differed in both the nm and μηι scales as a result of the different organisation and structural evolution of the clay layers. Furthermore, four different methods, defined as method A, Β etc, were tested for restoring the cation exchange capacity (CEC) of the thus prepared PILCs. The most effective proved method D which involved exposing the material to ammonia fumes followed by soaking in a sodium alkaline solution.Samples of ATHINA-A and AZA-A were used in the in vitro sorption experiments under static conditions. Kinetic experiments were analysed through the laws of isotopie exchange and film diffusion as the rate-determining step. Sorption rates of the order of hours were observed. The application of a phenomenological model developed for the interpretation of the results obtained in these experiments (NPL model), determined the selectivity coefficients of Cs or Sr over competitive cations of Na, Κ, Η and Ca. The observed PILCs' selectivity was interpreted through the electrostatic interaction of the cations and the negative charged clay layers. Furthermore, the influence of pH, solution volume and temperature in radiostrontium sorption was studied while, in experiments with cation concentrations simulating rumen liquid, the NPL model predicted the observed strontium sorption percentages. Comparison of PILCs with cyanoferrates (AFCF) revealed the reasons of AFCF effectiveness in cesium sorption and PILC superiority in strontium sorption. An unsuccessful in vivo experiment involving PILC administration to ruminants was attributed mainly to the limited time of contact between the material and strontium in the digestive tract of the animals and the insufficient material CEC restoration (method A).The materials AZA-D and FRAZA-D were used in cation exchange experiments. Cesium or strontium uptake was faster in FRAZA-D than AZA-D under both static and dynamic experimental conditions. This result was attributed to the different microscopic and macroscopic structure that materials possessed. Moreover, the separation factors Sr-Na and Sr-Ca were determined for FRAZA-D with values greater than 40 and 2 respectively. The selectivity of FRAZA-D for strontium sorption over several monovalent and divalent competitive cations was further confirmed in experiments with rumen liquid. The results derived from FRAZA-D administration in ruminants for strontium and cesium sorption were positive and promising for further investigation and research.