Chemical treatment of GaAs with sulfur-containing compounds has been shown to improve GaAs surface electronic properties. There is still considerable controversy, however, regarding the chemical nature of the surface film which results from the sulfidation, and of the basis of the electronic improvement and of the decay in the improved electronic properties with time. We have investigated the surface chemistry of the chemical sulfidation treatment of GaAs with Na2S.9H2O and (NH4)2S. Using surface infrared reflection spectroscopy (SIRS) and x-ray photoelectron spectroscopy (XPS), we have studied the GaAs surface and its behavior with time after such treatments. Results show that both of these sulfidation treatments removed the chemical oxide of GaAs, leaving behind a thin film on the surface. XPS results show that the Ga-O and As-O peaks were removed after treatment and that As-S and no Ga-S peaks were formed. Infrared results show that the film deposited after Na2S.9H2O treatment slowly reacted in air to form sodium carbonate and rhombic sulfur. In addition, this film contains compounds with sulfur-oxygen bonds, which most likely were arsenic sulfate, sulfite, and thiosulfate salts. The film deposited on the (NH4)2Streated GaAs surface was identified as ammonium thiosulfate and slowly decomposed with time. Rinsing with water removed the thin film formed after either sulfidation treatment.