Sulfur, strontium, and lead isotopic analyses were performed on chimneys from the hydrothermal vent fields (the PACMANUS and Susu knolls) in the eastern Manus back arc basin. Lead isotope composition varies in a narrow range (206Pb/204Pb = 18.75--18.78, 207Pb/204Pb = 15.51--15.54, and 208Pb/204Pb = 38.31--38.40). Strontium isotopic ratios (87Sr/86Sr) of sulfate minerals vary from 0.7051 to 0.7077, falling between those of seawater (0.7090) and the hydrothermal fluid end-member (0.7050). The δ34S values of the chimneys are -8.0 to -3.9? for the Susu knolls sulfide, +1.0 to +4.3? for the PACMANUS sulfides, and +7.4 to +20.6? for sulfate. The Susu knolls sulfides have the lowest values so far reported for volcanic-hosted massive sulfides from modern seafloor. The low δ34S values of sulfates and sulfides cannot be explained by a simple seawater circulation model without an additional source of light sulfur. This is also supported by the presence of sulfate in hydrothermal end-member fluid indicated by sulfur and strontium isotopic ratios in chimney sulfates. Biogenic sulfur and/or oxidation of H2S after boiling are excluded from the possible sources of light sulfur because of the lack of sediment cover and nonequilibrium in isotopic fractionation by boiling at seafloor. Disproportionation of magmatic SO2, introduced into hydrothermal system through preeruptive degassing, is the most plausible mechanism for the observed low δ34S values. The lighter δ34S values and more radiogenic lead isotope compositions of the Susu knolls samples indicate that magmatic degassing was more vigorous at the Susu knolls than PACMANUS.