Previously it has been shown that the electronic surface properties of GaAs can be improved by photochemical treatment in water. If this photowashing technique is carried out with intense white light, oxides several hundred angstroms thick can be grown. This paper reports the structure and composition of this photowashed oxide and one grown by soaking in stagnant water in low light. The oxide was determined by TEM cross sections to be highly porous, but with thin continuous oxide layers both at the surface and at the oxide/GaAs interface. The oxide is composed of Ga2O3with a low concentration of As2O3. The layer is primarily a fine grain Ga oxide crystal with a structure which appears different from the common forms of Ga2O3.