Second harmonic generation from the interface between GaAs(111)-B and an aqueous electrolyte has been investigated at a long wavelength (1064 nm) and at two shorter ones (865 and 815 nm). The signal showed the symmetry of the crystal; the isotropic amplitude was found to vary substantially with the electrode potential, indicating that a major part of the signal is generated near the interface. The potential dependence was stronger at shorter wavelengths. Impedance spectroscopy revealed the existence of surface states, which may affect the magnitude of the signal. (C) 2004 Elsevier B.V. All rights reserved.