A method for the determination of reduced sulfur species in inclusion fluids is described. Inclusion fluids are released by decrepitation in a nitrogen stream, the sulfur species are trapped in a 0.1 M NaOH solution and subsequently determined fluorometrically. Method blanks are low (ca. 3 ng S) compared to yields from vein quartz samples (80-6,700 ng S); the detection limit for reduced S is effectively set by this blank and corresponds to ca. 20 ppm S in the original inclusion fluid for a typical 0.5-g sample yielding a total of 1 mg of volatiles. The method is sensitive to S present as H 2 S or COS but not CS 2 or SO 2 .Analyses of inclusion fluids from unmineralized feeder veins in the Dolgellau gold belt, north Wales, indicate that Au(HS) (super -) 2 predominated over AuCl (super -) 2 as the major gold-transporting complex.