The ternary semiconductor GaInP2 (or Ga0.5In0.5P) was studied as a single-crystal electrode in contact with various aqueous electrolyte solutions (pH range 2.0-12.0). The photoelectrochemical properties of the epitaxially grown GaInP2 layers were characterized using techniques of current-voltage, capacitance-voltage and photocurrent spectroscopy. The results of capacitance-voltage measurements carried out at discrete frequencies from 100 Hz to 10 kHz demonstrate the presence of a depletion zone over the voltage range studied. This allowed the experimental determination of both the flat-band potential and the energetic positions of the band edges for both n- and p-doped GaInP2 at the semiconductorelectrolyte interface. These studies reveal the applicability of this relatively new material for the photoelectrochemical decomposition of water.