InterstitialY2Fe17−xMxCycompounds, withM=Alor Si andx⩽2,y⩽1,were investigated by57FeMo¨ssbauer spectroscopy at room temperature. In the analysis of the spectra, the preferential occupation by Al or Si of the Fe(12k)sites in the hexagonalR2Fe17phase was taken into account. The hyperfine parameters variations are discussed on the grounds of local crystallographic details at the inequivalent iron sites, altered by the composition modifications. The low values of the isomer shifts and quadrupole interactions suggest small electronic charge redistribution inY2Fe17−xMxAyas compared toY2Fe17.However, the isomer shift variations upon Wigner–Seitz cell volume changes evidence for screening and electron transfer effects. In order to provide a more consistent picture of the local effects of interstitial versus substitutional atoms inR2(Fe,M)17Aycompounds, the discussion of the present results is extended to57FeMo¨ssbauer data obtained on the parent compounds and their nitrides. ©1997 American Institute of Physics.