[en] Impurity effects are discussed from the viewpoint of intergranular stress corrosion cracking (IGSCC) of sensitized austenitic stainless steels in simulated reactor water. By using constant elongation rate tensile (CERT) testing it is demonstrated that low concentrations of certain, commonly encountered impurities may have a large effect on the tendency for IGSCC. This is especially true for sulfate and chloride ions and also, but to a smaller degree, for nitrate and carbonate. The effect of chloride is enhanced by copper ions and by hydrogen peroxide in start-up environment. (author)