Some different types of organic substances (an acetylenic derivative, S- or N-containing compounds) were tested as inhibitors for the intergranular (IG) corrosion of a sensitized AISI 304 stainless steel, in diluted sulphuric or sulphamic acid solutions at 70 °C. The sole compounds able to stop the IG attack were those with a S atom with electronic lone pair free. These inhibitors perforrmed their action by reducing the differences in the corrosion rates between the chromium depleted grain-boundaries and the matrix. The influence exerted by HS- ion on the corrosion process was evaluated, since some of the efficient inhibitors could decompose with the formation of HS- ions (or H2S).