We report a systematic x-ray photoemission study of the alloys ${\mathrm{Sm}}_{1-x}{\mathrm{Gd}}_{x}S$ for $0\le x\le 15.5\%$ which exhibit a first-order, black-to-gold phase transition, at $x_c\sim 16\%$, analogous to the one reported for SmS under pressure. We show that substitutional Gd produces a shift of the $4f$ levels of neighboring Sm atoms of about 0.5 eV, which to good approximation is independent of $x$. This allows us to obtain a reasonable fit to the ${\mathrm{Sm}}^{2+}$ → ${\mathrm{Sm}}^{3+}$ photoemission line shape using a model which has some similarities with the Jaccarino-Walker model for magnetic alloys. It assumes the existence of only two different kinds of ${\mathrm{Sm}}^{2+}$ ions in ${\mathrm{Sm}}_{1-x}{\mathrm{Gd}}_{x}S$ for $x\lesssim 16\%$. We discuss possible reasons for the fact that Gd does not show the usual clustering effects known to occur in many substitutional rare-earth alloys. Lattice-constant measurements for various substituents further illustrate the importance of electronic effects in the phase transition.

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