Proton‐induced characteristic X rays (1–10 keV) have been used to study,in situ, the alloy composition of 304L stainless steel surfaces as a function of the vacuum annealing temperature. Effects of 15‐min isochronal vacuum anneals between 700 and 1450°K were determined by probing to depths of about 1 &mgr;m with 150‐ to 400‐keV protons. In particular, chromium depletion was significant above 1200°K. For 200‐keV protons, the Cr(K&agr;)/Fe(K&agr;) ratio decreased from about 0. 8 for 700–1200°K to 0. 19 at 1450°K; this corresponds to a decrease in the chromium content from 19 to about 6 wt%.