The leaching of solid inorganic waste from loaded concrete or cement by incoming water can be described in terms of a “steady-state” outward diffusion of the saturated solution, formed inside the pores. Expressions are derived for the amount of leached material as a function of the concentration of this saturated solution, the specific load, the porosity and the retardation by the tortuosity of the pores. As experiments on leaching start with a dry test specimen, a correction for the flow of the incoming water is necessary. The porosity and the retardation by the tortuosity of the pores, compared to diffusion in a dilute aqueous solution, can be determined by combining two radiotracer experiments: That on leaching of test specimens with measurements on diffusion through a thin plane sheet of loaded concrete or cement, using HTO as the radiotracer. Numerical evaluation of the derived equations permits a prediction of long-term leaching behaviour. Assuming a specific load of = 10% waste material and moderate solubilities, the “effective” diffusion coefficients in present concrete formulations are of the order of 10−17–10−15m2·s−1.