Hydrothermal gases from shallow seafloor vents in the Bay of Plenty, New Zealand contain CO 2, CH 4, and the higher gaseous hydrocarbons up to i-, n-C 4H 10. The gases are similar to those discharged at fumaroles on the nearby White Island. Carbon isotope compositions for CO 2 fall between −3.4‰ and −5.5‰ PDB and reflect a shallow magmatic carbon source. The δ 13C values of CH 4 range from −24.6‰ to −28.9‰ PDB and the δD values vary between −122‰ and −135‰ SMOW. The CH 4 isotope values and the presence of the higher hydrocarbon compounds such as C 2H 6 and C 3H 8 with δ 13C values near −20‰ PDB suggest hydrocarbon production by high-temperature maturation of sedimentary organic matter and mixing (∼1:1) of the thermogenic CH 4 with abiogenic CH 4. Long-chained hydrocarbons occur in dredged samples close to the active vents. Their n-alkane distribution has a high to moderate odd–even predominance and an extensive hopane series, indicative of higher land-plant waxes and prokaryotic membranes in the source. Substantial amounts of unsubstituted polynuclear aromatic hydrocarbons (PAH) mark the transition from aliphatic- to aromatic-dominated bitumens, consistent with extensive source maturation resulting from thermal stress. The bitumens are interpreted as pyrolysates derived from buried near-coastal vegetation and terrestrial detritus under various thermal regimes, mixed with immature seafloor organic matter.