Structural changes occurring during the hydrogen-assisted pyrite-pyrrhotite transition are characterized over the temperature range 400–600 °C. Newly developed pressure-temperature microscopy gave in-situ observations of pyrite decomposition at elevated temperatures and gas pressures. Composition of pyrrhotites was measured by X-ray diffraction and correlated with krypton surface areas. Surface areas increased with an increase in pyrite reduction due to a decrease in molar volume accompanied by the development of porosity. Increases in specific surface area are inversely proportional to the reduction temperature. Heats of adsorption of krypton on pyrite increase linearly with increases in the relative amount of the (100) crystallographic faces.