We present new isotope (C, He, Ne, Ar, partly N) and compositional data from the free gas phase of fourteen degassing sites in the westernmost part of the Pannonian Basin near the Austria/Slovenia borderline. Based on these data, the origin of the gases and the degree of modification of the gas signatures due to interaction processes during migration are evaluated. The isotope signatures indicate an origin of helium and CO₂ predominantly in the subcontinental mantle. Measured ³He/⁴He ratios from 4.95 to 6.32 Ra include the highest ones recorded in the whole Pannonian Basin system. Only at three locations in the periphery of the degassing center, a substantial admixture of crustal helium was found. The CO₂ in the mofette gases and at the sites with the highest ³He/⁴He ratios (~ 6.3 Ra) is characterized by δ¹³C values of − 3.5‰. In comparison with MORB (Mid-ocean Ridge Basalt), it is thus slightly enriched in ¹³C. The ³He/⁴He isotope ratios within the range typical for the subcontinental lithospheric mantle (SCLM) point to a fast, localized fluid transport from the magmatic reservoir to the surface.

There are only few sites in European non-active volcanic regions where free gases with unmodified SCLM helium isotope signature escape at the surface. A comparison of the elemental and isotopic geochemical characteristics of gases with SCLM-helium signature from four different regions (Massif Central/France, Eifel/Germany, Eger Rift/Czech Republic and the westernmost part of the Pannonian Basin system) indicates that the European SCLM in general is characterized by a reservoir more enriched in ¹³C compared to MORB.