A systematic investigation of 2p core-level X-ray photoemission spectra (2pXPS) of a series of transition metal (TM) sesquioxides M2O3 (M = Ti, V, Cr, Mn, Fe) is reported. Theoretical analysis was carried out by means of an MO6 cluster model with the full multiplet structure of the M ion and the calculated results of the 2pXPS are in good agreement with the experimental data. Moreover, theoretical and experimental investigations of SpXPS, SsXPS, valence XPS and bremsstrahlung isochromat spectra (BIS) of Cr2O3 were also performed and all of these experimental data are satisfactorily reproduced with the cluster model approach. The character of the insulating energy gap of these M2O3 compounds was also studied based on the spectral analysis. It is shown that the M2O3 compounds from Ti2O3 to Mn2O3 are classified as intermediate-type insulators between charge transfer (CT) and Mott-Hubbard (MH) insulators, and Fe2O3 is classified as a CT insulator.