(NH4)2Sx‐treated GaAs (100) surfaces were heated in an ultrahigh vacuum. Reflection high‐energy electron diffraction (RHEED) and Ga, As, S, and O x‐ray photoelectron spectroscopic (XPS) changes were observed. The sulfide‐treated surface showed a streaky 1×1 RHEED pattern without heating. A 2×1 RHEED pattern appeared during heating to 260 and 420 °C. At these temperatures, the S XPS peak was still observed. The 2×1 pattern is thought to be S induced. On the (NH4)2Sx‐treated surface, no oxidized As XPS signal was observed. Moreover, the O XPS peak disappeared rapidly during the heating above 260 °C. These results suggest that the 2×1 S structure caused the GaAs (100) surface passivation.