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Two surveys of soil-gas and diffuse flux of CO2 and CH4 were carried out at Ustica Island (southern Tyrrhenian back-arc basin) in 1997 and 1999 to examine the degassing role of fault and fracture systems, as local evidence of wider structures of regional importance. Both surveys show that the Arso fault, crossing the island, is the main degassing structure. Fault-linked positive CH4 fluxes confirm that methanotrophic consumption in soil is higher in warmer periods but seems to be insufficient for consuming all leaking geologic CH4.Chemical and isotopic analyses of soil gases at higher degassing sites show that despite the extensive atmospheric contamination, He concentrations are always above the atmospheric level. 3He/4He isotopic ratios also suggest that a contribution of a deep-originated component is still detectable on the island.