Quenching experiments were used to define the thermal stabilities of phase assemblages in the system Ag-As-S. Extensive liquid immiscibility occurs in this system at high temperatures and immiscible liquids are present even below 400 degrees C. Five ternary solid phases were encountered: proustite and xanthoconite (both Ag 3 AsS 3 ), smithite and trechmannite (both AgAsS 2 ), and Ag 7 AsS 6 . The phase relations of proustite were studied in detail. The maximum temperatures at which proustite can coexist with either Ag 7 AsS 6 , Ag 2 S, As, or smithite are, respectively, 480+ or -2 degrees , 469+ or -3 degrees , 452+ or -2 degrees , and 409+ or -4 degrees C. These minimum-melting temperatures define upper-temperature limits (un-corrected for confining pressures) for deposition of corresponding assemblages in ores. The xanthoconite --> proustite transition occurs between 180 degrees and 202 degrees C (confirming previous data) and the assemblage proustite + silver is unstable relative to Ag 2 S + arsenic when heated above about 190 degrees C. Both reactions provide useful indicators for geothermometry. Equilibrium assemblages of smithite + realgar and smithite + orpiment could not be synthesized, although both assemblages occur in nature; phase relations show that the AgAsS 2 modification associated with either realgar or orpiment would be trechmannite if equilibrium were maintained during low-pressure deposition and subsequent cooling.