Unpinning of the Fermi level on GaAs (100) surfaces by photochemical reactions resulting from simultaneous exposure of specimens to flowing water and light was recently reported. We discuss here a series of experiments carried out to provide further information on the changes in surface electronic structure responsible for unpinning of the Fermi level under these conditions. The present work supports the conclusion that the surface states which pin the Fermi level are associated with elemental arsenic and arsenic sesquioxide (As2O3). Effects of each of these two species on pinning are distinguished experimentally. We find that, in addition to photochemical reactions, exposure to flowing water alone can result in Fermi level unpinning under certain conditions. The oxygen content of the wash water and the specimen preparation are shown to be important variables.