The rates at which strontium, cesium, plutonium, and curium are leached from hydrofracture grout (a modified cement) were measured. These studies utilized the test method proposed by the International Atomic Energy Agency or a modification that exposed smaller specimens with a higher surface-to-volume ratio to a larger volume of leachant. The fraction of an isotope leached varied with the square root of time if the leachant was replaced more frequently than once per day, but was inhibited or depressed if replacement was made less often. The amount of strontium or cesium leached from the grout varied directly with the degree of drying during curing and inversely with the time of curing. Of the clay additives studied for enhancing cesium retention, Grundite (while satisfactory) was the least effective. In general, the isotope leach rate followed the order: Cs > Sr > Cm > Pu. The amount of an isotope leached as a function of time depended on the composition of the leachant and varied in the order: distilled water > tap water > grout water. Concentrating the waste by a factor of up to 4 (prior to incorporation into a grout) had little effect on the leach rate of either strontium or cesium. A comparison of the leach data for the grout with results reported previously by other investigators for other products indicates that the grout can provide leach rates comparable to those obtained for wastes incorporated into borosilicate glass. Theoretical relationships that consider diffusion and instantaneous reaction (an equilibrium or time-independent relationship between mobile and immobile forms of a species) were found to be in good agreement with the data for the 28-day-cured grout when the leachant was initially replaced twice per day. The credibility of laboratory results with simulated waste was substantiated by a short-term continuous leach test made on a fragment of a core sample of actual hydrofracture grout. The modified effective diffusivities [10−11 to 10−10 cm2/s (10−9 to 10−8 mm2/s), equivalent to a leach rate of the order of 10−7 g/(cm2 day) (10−9 g/mm2 · day)] for strontium and cesium calculated from these data are comparable to those obtained with specimens prepared in the laboratory.