The hydrogen-reduction of spheroidal pyrrhotite particles, varying in diameter from about 0.4 to 1.6 cm, was investigated at 600, 800, and 900°C. Microscopic examination of partially reduced samples showed a well-defined interface between unreacted pyrrhotite and a porous iron shell. The rate of reduction is controlled primarily by counter-current diffusion of H2 and H2S in the gas-film boundary layer and in the porous iron layer. The magnitude of the ratio of effective diffusivity to the molecular interdiffusivity for H2-H2S, derrived from the reduction data, suggests that the structure of the porous iron formed is similar to that of an idealized porous material.