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Interstitial modification of R2Fe17Zx is studied with R=Y, Sm, and Z=N, C, B, Si, and S. Equilibrium interstitial concentrations are calculated using a lattice gas model. For gas-solid reaction a low solubility is obtained at high temperatures, whereas a solid-solid reaction is predicted to yield R2Fe17Z1.5 in the high-temperature limit. Theoretical results are compared with data on Sm2Fe17Cx produced by solid-solid reaction at 500 °C. Volume expansions up to 4.8% were obtained, which corresponds to x≈2. The concentrations of interstitial boron and silicon are much lower.