Dissolved gold contents in chloride brines that were experimentally saturated with gold, magnetite, iron sulfides, orthoclase, and muscovite at 550 degrees to 725 degrees C and 100 to 400 megapascals are reported here. Microsamples of the fluid were isolated at the experimental temperature and pressure as fluid inclusions in quartz. Individual fluid inclusions were opened by Laser ablation and analyzed by inductively coupled plasma mass spectrometry. The results show that gold solubility as sulfide species in supercritical brines is far higher than previously supposed. Dissolved gases such as molecular hydrogen sulfide that are ineffective metal-complexing agents at Low pressures evidently become highly effective at high pressures.