Slag-bullion equilibration tests were carried out to establish tie- lines between lead-rich and oxysulfate phases at several combinations of temperature and so2 partial pressure. The results obtained using sampling and quenching techniques were in good agreement and indicated the oxysulfate phase deviated very little from being a pbo-pbso4 melt. This deviation became more and more appreciable with increasing so2 pressure and/or temperature. However, it was less acute than previously determined. An analytical solution was performed to establish equilibrium tie-lines between pb-s and pbo-pbso4 melts. The results were used to derive an equation for the standard free energy of formation of pbso4 (1) in the temperature range of 1200 to 1450 k. The umr version of solgasmix, stepsol, was used in conjunction with the above metal and slag activity models to establish the tie-lines and to extend the results to regions not tested experimentally. The results allowed consideration of possible direct-smelting processes for clean lead concentrates, such as a typical Missouri concentrate. Excellent agreement between model predictions and analytical solution was obtained, suggesting confidence in the usefulness of the model. There is, however, a lack of agreement between the experimental data and the model predictions. The sources of this disappointing problem are not fully understood at this point.