Existing thermodynamic data for the Fe-As-S-0 system were evaluted and predominance area diagrams for that system were constructed at 798 and 973 K. Isopleths for the As/S and As/O atomic ratios in the vapor phase have been added to the diagrams by solving the complex equilibria. These modified diagrams were used to evaluate the results of roasting both natural and synthetic arsenopyrite (FeAsS) in inert, reducing, and oxidizing atmospheres at 798 and 873 K. Conditions leading to the retention of As as As2O5(s) and FeAsO4(s) were also reviewed. The experimental results indicate that both reducing and oxidizing atmospheres are more effective in the removal of As than an inert atmosphere. In a reducing atmosphere arsenic sulfides are evolved and the percentage of As removal increases with decreasing PO 2The greatest percent of As removal occurred with highly oxidizing atmospheres which generated As4O6 vapor.