The origin of carbon-bearing compounds (CO(2), CH(4), C(2)-C(4) saturated hydrocarbons) and helium in the geothermal fluid of Larderello is investigated by means of the variations in concentration and isotopic composition. The CO(2) (delta(13)C from -1.4 to -7.1 parts per thousand versus V-PDB) is mainly of crustal origin. The carbon isotopes of methane (delta(13)C from -20.9 to -31.7 parts per thousand) and other hydrocarbons indicate a complex thermogenic origin. The temperatures obtained with the CH(4)-CO(2) isotope geothermometer are in rough agreement with those observed in deeper geothermal wells. The CH(4)/C(2)H(6) ratios show a tendency towards partial equilibrium with increasing temperature. He isotopes (R/R(A) from 0.5 to 3) indicate that although the major part of helium derives from crustal sources, a significant fraction of mantle helium is also present. Helium contamination by air, deducted from He/Ne ratios, is generally negligible. (c) 2005 CNR. Published by Elsevier Ltd. All rights reserved.