Two grades, a model 304H stainless steel and a X-750 superalloy, were tested at high temperature during slow strain rate tensile tests under secondary vacuum. The 304H stainless steel was tested at 680 degrees C and 3 x 10(-6) s(-1). The X-750 superalloy was tested at 750 degrees C and 3 x 10(-6) s(-1). The reduction of area and the ductility dip were characterised by tensile tests up to rupture. In the ranges of temperatures and strain rates applied, the fracture was brittle intergranular for both grades. The mechanisms of embrittlement were studied after interrupted tensile tests, after which some samples were broken in an Auger electron spectrometer. The segregation of S and P was then analysed at the grain facet scale. In the case of the 304H grade, the segregation of S and P was higher at the border of the facet. In the case of the X-750 grade, the segregation of S was located at the precipitates, and the segregation of P was located at the precipitates and at the bottom of intergranular microvoids. A layer by layer quantification model was used to estimate the fractional surface coverage of S and P. For the 304H, the embrittlement is assumed to arise from the opening of wedge cavities at triple points. For the X-750, the embrittlement is assumed to arise from the nucleation and growth of cavities at the intergranular precipitates.