Optically stimulated deep-level impedance spectroscopy is shown to be sensitive to transitions between deep-level states in an n-GaAs Schottky diode. The technique is based on interpretation of both the real and imaginary components of the impedance response over a continuous range of electrical frequencies under sub-bandgap illumination. The results obtained are compared to those by deep-level transient spectroscopy and thermally stimulated deep-level impedance spectroscopy. Optical energies associated with onset of electronic transitions are easily identified at low electrical frequencies and from regression to the resulting spectra of a model that accounts for the influence of deep-level states.