Electrolyte electroreflectance (EER) measurements at different modulation frequencies are carried out for the characterization of semiconductor \ electrolyte (SC \ EL) junctions. It is shown that the frequency dependence of the intensity and lineshape of EER signals is governed by the electrical impedance of the junction. Examples are given for n-RuS2 and n-GaAs electrodes in contact with aqueous electrolyte. At the n-RuS2 \ H2O interface, electroreflectance spectra reveal the presence of an interfacial layer, most probably of RuOx composition, which determines the behavior of this oxygen evolving electrode. Modulation of the electric field in this layer, and thus observation of its electroreflectance signal, requires the trapping of photogenerated holes at the RuS2 \ RuOx interface followed by recombination with conduction band electrons. At the n-GaAs \ H2O interface, the oscillatory reduction of H2O2 is followed through EER measurements. Oscillation in baseline, lineshape and peak-to-peak intensity of the signals is observed. Baseline oscillation can only be observed at low modulation frequency and is attributed to a surface layer, probably As-0. The lineshape reflects an oscillating inhomogeneity in the scl electric-field attributed to H+ intercalation in the near-surface of the GaAs electrode.