The gas-phase interstitial modification of rare-earth intermetallics is studied. Net reaction energies for nitrogen in Sm/sub 2/Fe/sub 17/ and Nd(Fe/sub 11/Tl) are U/sub 0/=-57 kJ/mole and U/sub 0/=-51 kJ/mole, respectively. The equilibrium nitrogen concentration is calculated as function of temperature and gas pressure using a simple lattice gas model. For nitrogen in Sm/sub 2/Fe/sub 17/, refined diffusion parameters D/sub 0/(N)=1.02 mm/sup 2//s and E/sub a/(N)=133 kJ/mole, determined by thermoplexic analysis of the initial stage of nitrogen absorption, are used to calculate nitrogen profiles and the time dependence of the mean nitrogen content during nitrogenation. Similar values are obtained for nitrogen in Nd(Fe/sub 11/Tl), whereas the activation energies for hydrogen in Sm/sub 2/Fe/sub 17/ and Nd(Fe/sub 11/Tl) are 31 kJ/mole and 45 kJ/mole, respectively. The elastic stress and strain profiles during nitrogenation are calculated. Important results are a large uniaxial strain near the surface of nonuniformly nitrided particles, and core expansion even in the absence of any nitrogen there. >