Strontium hexaferrite powder has been treated in nitrogen, hydrogen and carbon atmospheres. The results show that the phase composition and morphology, and hence, the magnetic properties of the strontium hexaferrite are affected significantly by these gas/vapour treatments. Generally, the coercivity decreased to below 0.8 kOe (regardless of the initial coercivity) and the magnetization at 14 kOe increased significantly, when strontium hexaferrite powder had been treated in a nitrogen, hydrogen or carbon atmosphere. However, it was found that a post-gas treatment of calcination in air, under appropriate conditions, resulted in a recovery of the hexaferrite structure (i.e. it is a reversible reaction). However, the particle/grain sizes of the calcined samples were significantly smaller than those of the non-treated samples, and it is believed that they were single domain particles/grains. In some cases, the coercivity increased by about 400%. The magnetization at 14 kOe and the remanence were either not affected or sometimes increased; magnetic measurements indicated a preferred orientation of the grains.