The conversion of semi‐insulating GaAs toptype as a result of heat treatment in H2was studied by photoluminescence (PL), secondary‐ion mass spectrometry (SIMS), and transport measurements. The SIMS measurements resulted in the direct chemical identification of Mn near the heated surface. The correlation of the SIMS profiles with the results of PL and transport measurements indicated that Mn acceptors are responsible for the type conversion, and that substantial concentrations of Mn(?1017/cm3) are found in thin (1–3 &mgr;m) layers near the surface. The results of studies of samples heated under several different conditions showed that the Mn layers were not introduced by contamination from external sources during heat treatment, but were probably due to the presence of a bulk Mn concentration (<3×1015/cm3): during heat treatment the Mn diffuses to the surface, probably assisted by the in diffusion of Ga vacancies, as suggested by Zucca. Photoluminescence profiling measurements and the correlation between the SIMS and PL results indicate that the 1.41‐eV PL band that is associated with the type‐converted surface is due to recombination at a Mn acceptor on a Ga site, and not at a next‐nearest‐neighbor arsenic vacancy‐amphoteric acceptor complex.