The Hydrogen Disproportionation Desorption Recrystallisation process (HDDR) is still used to achieve high coercive fine powders of Nd2Fe14B. Here we study the solid-gas reactions (hydrogenation and nitrogenation) on Sm-2(Fe1-xCox)(17) alloys. In order to insure stoichiometry of the Sm2M17 alloys along the thermal treatments, some iron has been substituted by cobalt (x = 0.3, 0.5). Fine particles were achieved by using a planetary miller, and sieved up to a size less than 10 mu m. Solid-gas reactions (Sm-2(Fe1-xCox)(17)/H-2, N-2) were analysed by using a thermomagnetic balance, the sample reacting under the flowing gas. Clear relationships between the particle sizes and the kinetics of reactions were found. The HDDR process was optimised for recrystallisation rate, plateau temperature and duration. High coercive Sm-2(Fe1-xCox)(17)N-y powders with H-cj up to 18 kOe were achieved.