The kinetics of the reactions at the GaAs-H3BO3 interface at pH 5 is modified by the presence of Co(II) ions in the solution. This effect is shown for oxygen reduction in the dark, at the surface of p-GaAs in aerated solutions: the rate of oxygen reduction is higher for higher Co(II) concentration. Photoanodic dissolution of n-GaAs in deaerated boric acid solution is also affected by Co(II) ions, which enhance the dissolution rate of the semiconductor (SC). It is suggested that a thin coherent oxide film is formed on GaAs in Co(II)-free boric acid solutions. In the presence of Co(II), the breakdown of the oxide allows oxygen adsorption on bare GaAs sites, and the kinetics of oxygen reduction is enhanced at p-GaAs. Breakdown of the oxide layer in the presence of Co(II) ions in the boric acid solution is also obvious during photodissolution of n-GaAs in the absence of oxygen.