We have studied 4f photoemission spectra of thulium monochalcogenides at excitation photon energies of the 3d-4f resonance region (hv = 1450-1520 eV). The experimental results are compared with theoretical calculations. The contributions from the 4f divalent part and the trivalent part in the 3d-4f resonance are more clearly distinguished and resolved than for the 4d-4f resonant photoemission. The calculated results are in good agreement with the experimental ones both for divalent and trivalent features of thulium ions. We have estimated the mean valence of TmS, TmSe and TmTe samples from 3d absorption spectra and from off-resonant photoemission spectra. The latter should reflect more bulk-sensitive information than that reported for the 4d-4f resonance region. It is concluded that the 3d-4f resonant photoemission is a more useful technique to investigate 4f electronic structures than the 4d-4f photoemission, when we perform the experiments with relatively higher energy resolution. (C) 1998 Elsevier Science B.V.