Passivating AsxSyphases that form on GaAs after reactions with aqueous sulfides [Na2S⋅9H2O and (NH4)2S] decompose in the presence of oxygen and light, producing a surface composed primarily of As2O3. X‐ray photoelectron spectroscopy (XPS) shows that (NH4)2S‐treated surfaces degrade in two steps with the initial AsxSy(present as a disulfide) degrading by S–S bond cleavage to yield As2O3and an As2S3phase like the one that forms on GaAs after Na2S⋅9H2O reactions. XPS reveals little decomposition of surface sulfides in the presence of light or oxygen alone, showing that GaAs passivation could be made long‐lived if either is rigorously excluded.