The durability of type K and H cement based grouts under conditions potentially found to be in high level nuclear waste repositories was studied. Tests have been carried out to determine the effects of temperature on the hydraulic conductivity and the leaching resistance of the grouts. Mea surements of merCil ry intrusion porosimetry and scanning electron microscopy with energy dispersive X-ray analysis have been used to investigate the changes in the pore structures of both grouts as f unction of leaching and permeating time. Type K and type H cement based grouts, made with low water to cemeniitious materials ratio, silica f ume, and superplasiicizer, were exposed to high temperature and pressure. Preliminaru results indica te that both hydraulic properties and leaching behavior of the grouts investigated are affected by the increase in temperature. However, data show that leaching rate and hydraulic conductivity of the grouts decrease with time. The results showed clearly that chemical reactions, presumably accelerated by tile elevated temperatures (100Β°C), led to the [orma tion of a precipitate ill the microcracks and on the surface of the leached specimens. This precipitate is likely the cause of the observed decrease in the hydraulic conductivity and leaching ra te.