Interstitial Sm–Fe-based nitrides with compositions close to that of Sm 2Fe 17N 3 are an interesting class of hard magnetic materials. In this series of experiments we characterised the nitrogenation process using a vibrating-sample magnetometer (VSM) and a thermogravimetric differential thermal analyser (TG/DTA). The VSM was fitted with a high-temperature furnace tube and a system for varying the atmosphere around the sample with different gases between 10 −2 mbar and 1 bar. The experiments involved two alloys, with the compositions Sm 13.7Fe 86.3 and Sm 13.8Fe 82.2Ta 4.0, and three different particle sizes, <2 mm, <200, and <15 μm. The results show that the nitriding process is very sensitive to the composition and the particle size, and that nitriding at higher temperatures can result in a nitrogen disproportionation and a loss of the Sm 2Fe 17 phase.