In this work electrochemical impedance spectroscopy was carried out on pre- and post-etched Ga0.5In0.5P (1.8-1.9 eV) as well as quinolinol-modified surfaces, over seven decades of frequencies. The spectra were examined in the Bode as well as complex impedance planes and deconvoluted using nonlinear-least-squares fitting to obtain equivalent circuits that isolated the space charge from the surface states. Chemical wet-etching of Ga0.5In0.5P was found to remove surface states as evidenced in the removal of the low frequency time constant from the spectra. Treatment with 8-quinolinol was found to shift the flat-band potential in the positive direction vs. she by about 300 mV in near neutral electrolytes.