We have applied x-ray absorption spectroscopy (XAS) in the region of the Tm MV edge to study the intermediate-valence behavior of the Tm ions in the series of mixed chalcogenides TmxY1-xSe, with 0.004x1. The clearly distinguishable spectral features resulting from divalent and trivalent Tm ionic states can be fitted with good accuracy to yield the mean Tm valence v. We discuss the precision of the resulting values critically and consider the surface sensitivity of the technique. The results for concentrated TmxSe are in good agreement with lattice-constant systematics. In the dilute materials, we confirm the intermediate valence of Tm down to x=0.4 at. % and find v to be nearly constant below x=20 at. %, but to be lower than in TmSe. © 1985 The American Physical Society.