A mathematical model to predict the release of toxic elements immobilized in different cementitious matrices is presented. This model has been obtained on the basis of previous experimental tests carried out adopting a dynamic leaching test proposed by the American Nuclear Society. During the tests, lithium and bivalent copper have been used as tracers; different types of cementitious matrices and lime-pozzolan mortars have been tested. To correlate the release of these two elements with the intrinsic properties of the matrix and of the immobilized element system, a statistical approach has been used. The final semiempirical model obtained has been very satisfactory for the prediction of contaminant loss from cementitious immobilizing matrices.