The authors have applied contactless electroreflectance (CER) spectroscopy to study the epi readiness of m-plane GaN substrates obtained by the ammonothermal method. It has been clearly observed that the CER resonance, which is related to the energy gap transition, appears for samples with a well-polished surface. The sharpness of this resonance is directly related to the surface quality. The broadening of energy gap transition can be used as a parameter to quantify the surface quality. For samples polished with optimal conditions, this broadening (gamma(pol)) is close to the broadening observed for the cleaved GaN surface (gamma(clev)) with m-plane orientation (150-190 vs 135 meV). The quality of the polishing process can be evaluated by analyzing the gamma(clev)/gamma(pol) ratio, where gamma(clev)/gamma(pol)=1 corresponds to an excellent polishing process. In the authors' case, this ratio has been determined to be close to 1 for well-polished samples. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3504359]