Electronic structures of thulium monochalcogenides (TmS,TmSe,TmTe) were studied both experimentally and theoretically using a resonant photoemission technique in the photon energy range around Tm 4d edge (100-190 eV). The experimental results show a large resonant enhancement of 4f photoelectron emission around the 4d threshold with sharper multiplet structures than those reported before. The calculated results are in good agreement with the experimental ones both for divalent and trivalent features of Tm ions. We observed different types of valence structures for TmS, TmSe and TmTe compounds, due to change in mean valence. We have estimated the mean valence of each sample from absorption spectra and from off-resonant photoemission spectra. The meaning of these values is discussed as compared with those for the values obtained from the measurements of magnetic susceptibilities.