The alpha-beta phase transition in MnAs thin (100 nm) films epitaxied on GaAs(001) is studied using optical spectroscopies. It is found that reflectance anisotropy spectra differ markedly for the two phases. Although no prominent feature appears in the spectrum related to the beta phase, the spectrum of the alpha phase exhibits two features centered at 2.9 and 5.0 eV. The origin of these spectral features has been further investigated using temperature-dependent differential reflectance. It is shown that the band centered at 2.9 eV can be assigned to interband transitions for light polarized along the c axis of alpha-MnAs. We show that reflectance anisotropy spectroscopy can be used to monitor the alpha-beta phase transition within the thin film. The observed spectral features and their evolution across the coexistence region are discussed in light of band-structure models in the literature. The data suggest that each phase retains its electronic structure in the coexistence region and that the transition between the alpha exchange-split band structure and the beta band structure is sharp.