Hydrogen decrepitation of Sm₂Fe₁₇ followed by treatment in ammonia produces interstitial nitrides with nitrogen concentrations greater than three per formula unit. Measurements of the lattice parameters and magnetic properties are consistent with interstitial nitrogen occupation of the 9e and either the 3b or 18g sites. The effect of the extra nitrogen on the iron sublattice is to reduce the overall moment, particularly of the 18h sites. The magnetocrystalline anisotropy constants K₁ and K₂ indicate a significant decrease in the easy-axis character compared to Sm₂Fe₁₇N₃ material.