Single p-type, GaAs crystals of high purity, Zn doped, were used to determine whether or not the inverse octahedral {ill} faces show potential differences and various rates of anodic dissolution. The Ga(lll}, As(ill}, (If0}, and (100} faces, were polished, etched, and etch-polished with concentrated H2SO4 ~ H=,O.~, and immersed in IN KOH. The Ga(lll} faces were found to be the most noble with respect to rest and anodic dissolution potentials. The potential difference between the inverse (lll} faces was as large as 0.14v for the rest and 0.123v for the dissolution potentials. The 4 anodic polarization curves gave nearly parallel Tafel lines, with a slope of 66.0 ~-1 mv/log i, up to current densities of 0.5 ma/cm 2. The rate of anodic dissolution of the As{lll} faces was 69 X as high as the inverse Ga(lll}. The activation energies of dissolution of all 4 faces were equal within experimental limits: 16.7 _ 0.7 kcal mole -I. It is concluded that the slow step in the dissolution of GaAs is a one electron discharge with subsequent steps leading to the formation of Ga (OH)3 to provide a protective coating not readily soluble in KOH. From this point of view all observed phenomena can be explained in a qualitative manner.Crystals without a symmetry center are polar; the polarity shows up through differences in chemical and physical behavior of the inverse planes of the respective crystals. The cubic HI-V semiconductor compounds of diamond-type structure belong to this class, of which the best known are GaAs and InSb. Their polarity is displayed by the behavior of the inverse octahedral planes III{lll} and V(lll}, e.g., GaAs by Ga{lll} and As{lll}.Reaction of III-V semiconductors in various media, with or without applied current have been studied by Gatos et aI. (1), Pleskov (2), Gerischer (3, 4), Harvey (5), Brummer (6), Arthur (7), and others (8). They found that the various crystal planes react differently in aqueous media, such that the III(lll} and V{lll} inverse planes can be distinguished by the formation of specific etch patterns (1, 3, 8). The distinction can also be made from etching rates (1) from the inclination of etch tunnels produced by an anodic current on the GaAs octahedral planes (the tunnels run perpendicularly to As{lll} and at an angle of 20 ~ to Ga(lll}) (8), and from LEED patterns (9). Furthermore, the two inverse planes exhibit different activation energies of oxygen desorption (as Ga20): 54• from the Ga(lll} and 42• kcal/mole from the As face. Since there is little transfer of charges over the valence bridges (9, 10-13), the Ga atoms on the III(lll} side retain their 3-valence electrons, while the As atoms on the inverse side retain 5. All this suggests that the two planes should also develop potential differences in electrolytes.However, in this respect there are major disagreements. Gatos et al. reported that group III{lll} planes exhibit more noble electrode potentials than group V(lll} surfaces: for instance InSb inverse octahedral planes show a potential difference of 75 and more mv (1)....