We have shown that the out-of-equilibrium hexagonal P6∕mmm phase adopts the stoichiometry Sm(Fe,Ga)9 in the Sm–Fe–Ga system. X-ray diffraction analysis has been combined with Mössbauer spectrometry. A specific program for the Wigner–Seitz cell (WSC) volume calculation of the equilibrium and of the metastable structure, deduced from the results given by the Rietveld analysis, was used. The Mössbauer spectra have been simulated with a model based on: (i) the multinomial law related to the statistical distribution of Ga and Sm vacancies obtained by the crystallographic approach, and (ii) the correlation between the WSC volume and the isomer shift of each inequivalent crystallographic site. The specific behavior of the 3g site isomer shift evolution corroborates the structure model. The Curie temperatures of the Sm(Fe,Ga)9 series are higher than those of the Sm2(Fe,Ga)17 alloys. Sm(Fe,Ga)9 represents an interesting host lattice for interstitial C or N atoms, with possible permanent magnet applications.