The effect of the metal to sulphur ratio in the nonstoichiometic compound, pyrrhotite, is related to the consumption of cyanide in gold leaching. Reactive pyrrhotite (Fe(1-X)S) is produced during the pyrolysis of refractory pyrite (FeS2). Iron vacancies give pyrrhotite its non stoichiometric nature and determine its fugacity and hence reactivity. Separate trends for cyanide consumption were found when pyrite was partially converted to pyrrhotite compared to where all the pyrite was converted to pyrrhotite. Samples that were partially converted to pyrrhotite, and hence remained in equilibrium with the unconverted pyrite, showed an increase in cyanide consumption with increasing metal to sulphur ratio. Samples in which all the pyrite was converted to pyrrhotite showed a decrease in cyanide consumption with an increasing metal to sulphur ratio. For all samples the higher the pyrolysis temperature the lower the cyanide consumption.