The electrochemical modulation reflectance spectra (EMRS) of MnO2 film electrodes in 1 mol l−1 NH4Cl solutions were obtained. An electrolytic cell was specially designed to meet both the requirements of the reflectance spectroscopy and the electrochemical measurements. The typical spectra of γ/(β, R*), γ and γ/α-MnO2 exhibited typical France–Keldysh oscillations. Hence, the energy gap (E g) value of these samples can be determined by means of the three-point method of Aspnes. The spectra of γ-MnO2 films with varying electrode potential have also been obtained, by which the flat band potential (ϕ fb) of γ-MnO2 can be located. This information plays an important role in studies on the semiconductor behavior of γ-MnO2 and on the structure of the γ-MnO2 ∣electrolyte interface.