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Measured magnetization and 57Fe hyperfine fields at T∼0 K for Y2Fe17 and Y2Fe17Z3- δ , with Z=H, C, or N and δ∼0.5, are analyzed to determine the influence of the interstitial atoms on the 3d magnetism. All are weak ferromagnets with a nearly-full 3d subband, although when Z=N, the magnetic moment of 38.1μB is very close to the fully saturated value of 39.4μB. On a local scale, the order of hyperfine fields Bhf4f (=35 T)> Bhf6g (=33 T)> Bhf12j (=30 T)> Bhf12k (=28 T) is different from that of the local magnetic moments given by several recent band calculations for Y2Fe17, μ4f (=2.5μB)>μ12j (=2.2μB)>μ12k (=2.0μB)>μ6g (=1.9 μB), reflecting a large 4s-transferred hyperfine field at 6g sites which have very short Fe-Fe distances. The volume of samples with Z=C and N are identical, and so the chemical effects of the interstitials can be distinguished; both the local magnetic moment and hyperfine fields are systematically smaller in the carbide than the nitride by 5% and 12%, respectively.