To more completely assess the primary origin of IPGE alloys, we conducted experiments to evaluate the effects of T, fO₂, fS₂ and melt composition on the solubility of Ru in molten Fe-Ni-sulfide. Fe-Ni-S melt + Ru were held in olivine crucibles, and experiments were done in a vertical-tube gas-mixing furnace at 1200-1400C for 1-5 days. At constant fO₂ and fS₂, Ru solubility is prograde with T, but if fO₂ is varied parallel to FMQ, solubility is retrograde. At log fS₂ of -1.8, Ru solubility decreases from >15 wt% at log fO₂ of -11, to ~0.3 wt% at log fO₂ of -8. At a log fO₂ of -9, a similar reduction in Ru solubility occurred as log fS₂ decreased from -1 to -2.7. Substitution of Ni for Fe in the sulfide results in an increase in Ru solubility, with values ranging from ~3 wt% at Fe/Ni of 36 to ~10 wt% at Fe/Ni of 6 (log fO₂, fS₂ of -9.5, -1.8). Dilution of Ru with a 1:1 mix of Os + Ir results in a 4 and 10-fold decrease in melt Ru content for alloys with 50 and 25 wt% Ru, respectively. For the fO₂-fS₂ conditions required for sulfide liquid saturation in natural basaltic magmas, pure Ru solubility is expected to exceed 10 wt%, and dilution by Os + Ir is still likely to require wt% levels of Ru for IPGE alloy saturation. Since Ru abundances of ore-grade massive sulfide is <50 ppm, our results would preclude IPGE alloy saturation in the presence of immiscible sulfide liquid. The negative fO₂ dependence of Ru solubility in sulfide liquid is opposite that for Ru (and other PGEs) in silicate melt, suggesting that a decrease in fO₂ will favor the partitioning of Ru into the sulfide liquid. This effect may be partially offset, however, by concurrent changes in sulfide melt Fe/Ni ratio.