The effect of heat treatments on the interstitial nitrides and carbides prepared from Sm 2(Fe,M) 17 and Sm 2Fe 15Ga 2C 2 by gas–solid reaction (GSR) was investigated in order to understand more about the influence of the substitutions M=Ga, Al and Si on the thermostability of these compounds. X-ray diffraction measurements of the heat-treated samples have shown that all of these substitutions improve the thermostability of the Th 2Zn 17-type interstitial structure, with Al having the smallest influence and Si and Ga having a stronger influence on the thermostability. Contrary to the nitrides, the carbides prepared by GSR can be significantly stabilized with an increasing amount of the stabilizing elements. Already for a small amount of the substitution element, e.g. Sm 2Fe 16.5Ga 0.5, the 2 : 17-type interstitial phase was stabilized up to 750°C. Carburization at temperatures above 500°C was possible for the stabilized 2 : 17 compounds making possible much shorter carburization times for high C contents. Investigations on Sm 2Fe 15Ga 2C y have revealed a dependence of the thermostability on the amount of absorbed C and on the processing route for the interstitial modification by GSR. A coercivity μ 0J H c up to about 2.8 T was obtained for Sm 2Fe 15Ga 2C 2N showing the magnetic potential of these modified compounds.