The chemical and morphological transformation of ferrihydrite during calcination and subsequent sulphidation is addressed using in situ techniques (BET, XPS, TPS). The sulphidation process (in 10% H2S/H-2) appears to be diffusion controlled. Surface and bulk composition after sulphidation are in close agreement (FeS1.4 - pyrrhotite) and suggest the presence of excess sulphur. This excess sulphur can be removed via simple hydrogenation to yield FeS (troilite). Strong structural collapse occurs during sulphidation and is caused by agglomeration/sintering of primary crystallites. The BET surface area of the sulphided material is found to be very low (2 m(2)/g).