A special field of application of cementitious materials is using cements for immobilisation of low and medium level radioactive wastes. Clarification of mechanisms of binding is complicated by the multicomponent nature of the solidifying matrix. In the present work, interest is turned to one of the most difficult to confine, long half-life isotopes, the caesium isotope. The cement matrix for solidification of the radioactive waste can be considered, with simplification, as a CaO-SiO2-Al2O3-H2O system. The various compositions of hydrated cementitious assemblages were investigated with respect to their Cs sorption by measuring the Cs distribution ratios (Rd,) therein. Trends in sorption properties were detected, and the section of theternary phase diagram with the best performance was identified.