Five samples of hydrothermal fluids from two vent areas on the southern Juan de Fuca Ridge were analyzed for dissolved gases. Concentrations in the end-member hydrothermal fluid of H2 (270--527 μmol/kg), CH4 (82--118 μmol/kg), and CO2 (3920--4460 μmol/kg) are well above values in ambient seawater and are similar to concentrations reported for other ridge crest hydrothermal systems. The carbon isotopic ratios of CH4 (Δ13C=-17.8 to -20.8) and CO2 (Δ13C=-3.6 to -4.7) suggest that at least some of the CH4 and CO2 in the fluids is basalt-derived. The range of Δ13C values for the basalt-derived CO2 is -6.8 to -9.7, calculated by assuming conservation of recharge &Sgr;CO2 during hydrothermal circulation. Apparent temperature of equilibration between the CH4 and the basalt-derived CO2 range from 640¿C to 750¿C. Small amounts of ethane (C2H6/CH4≂0.9¿10-3 -2.2¿10-3), propane, and butane detected in the samples may also have formed in the basalt. One sample of almost pure 95.5%) hydrothermal fluid contained a significant fraction, up to 63% and 74%, respectively, of the recharge Ar and N2. This suggests that the fluid has not undergone extensive vapor-liquid phase separation. ¿ American Geophysical Union 1988