The desulfurization rate in a Peirce–Smith converter (which works as a well-mixed reactor) was obtained during the copper blow. Conversion data of the copper sulfide as a function of the operation time were correlated distinguishing between two kinetic steps; in both cases, the conversion was proportional to time. The variation of the matte temperature was not influential, so the chemical resistance in the bulk of the melt can be considered negligible. The variation in the global volumetric coefficient deduced from the experimental data was justified by a model having two resistances to mass transfer, the gas-side resistance being constant and the liquid-side resistance increasing when the sulfide concentration decreased.