Chemical treatments of GaAs with sulfur containing media have 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. We have investigated the surface chemistry of GaAs after chemical and electrochemical treatments with Na2S and (NH4)2S solutions using surface infrared reflection spectroscopy and x‐ray photoelectron spectroscopy. We have found that these sulfidation treatments removed the GaAs oxide and deposited a film on the surface which includes various arsenic sulfide and sulfur–oxygen species. No arsenic oxide, nor gallium oxide or sulfide after the treatments is detected. The arsenic sulfide observed after chemical (NH4)2S treatment is more stable against oxidation in air than the one observed after chemical Na2S treatment. Both of these sulfides, however, are removed with water rinsing. The arsenic sulfide phases formed after electrochemical sulfidation with both Na2S and (NH4)2S solutions are not removed with water rinsing. This anodic sulfidation treatment may be promising as a new method to passivate and improve the GaAs surface electronic properties.