A study on the pyrrhotite chemical leaching by ferric iron is reported in this work. The main aim of the work was the determination of stoichiometry and kinetics of the indirect mechanism during bioleaching of pyrrhotite. Experimental results have shown that the sulfur moiety of pyrrhotite is oxidized incompletely by ferric iron to elemental sulfur. A mathematical model able to predict the process kinetics has been developed from the unreacted core model, considering a dependence of the activation energy on the pyrrhotite conversion (Brittan, 1975).