Rates of sulfide scale formation on pure iron and a series of chromium steels have been measured over the temperature range mixtures at a total pressure of 600 psi with an partial pressure of 0.6 psi for exposure periods up to 500 hr. A mechanism of scaling, involving the depletion of a barrier scale layer by recrystallization and grain growth processes, is proposed and substantiated by laboratory studies of the structural properties and mode of formation of scales grown on the various materials studied. A single kinetic equation, based on this mechanism, is shown to satisfy the rate curves over the complete range of conditions studied. A solid defect structure, protons trapped at metal ion vacancies, is suggested to explain the effect of hydrogen on the scaling process. The term "E" center is suggested to describe such a lattice irregularity.