Abstract: Before or during their solidification in a matrix, the radioactive ion exchange resin particles, which can be anion and/or cation resins, treated with an additive, which has inorganic or organic anions or cations or organically anion active or cation active components and is chosen in such a manner that the treatment has the effect to reduce the swelling behavior of the resin particles and preferably produces a permanent shrinkage of the resin particles. This change of the resin particles can also be attained or supported by a thermal treatment.A matrix contains ion exchange resin particles which are treated in such a manner shows an improved water resistance which, contrary to the current state of the art, will also remain assured when the matrix is dried and then again stored in water. In addition, it is possible to accommodate a larger amount of treated resin particles in a volume, as compared to untreated resin particles.