Electroreflectance spectra from a highly doped n-GaAs sample in an alkaline electrolyte have been modelled, taking into account both Franz–Keldysh and bandfilling (Moss–Burstein) effects. Comparisons are made between theory and experiment, and the evolution of spectra calculated for a wide range of space-charge potentials is found to follow accurately that of the experimental spectra with applied potential. The proportion of the interfacial a.c. and d.c. potential differences accommodated outside the semiconductor can also be obtained from the analysis, allowing us to extract the Fermi level pinning and to associate this with possible surface chemical reactions. The importance of heavy-doping effects is also briefly discussed, and the inclusion of bandfilling in the modelling of the electroreflectance spectra of highly doped n-GaAs is shown to be essential if a quantitative fit is to be obtained.