Gas flux measurements have for the first time been taken from vents and soil of eastern Romania mud volcanoes, the largest geological structures in Europe releasing methane into the atmosphere. In the quiescent phase, the methane emission from single vents is up to 28 t yr−1. Diffuse soil microseepage is of the order of 102−105 mg m−2 day−1. A total output of at least 1200 tonnes of CH4 per year can be conservatively estimated over the area investigated alone (∼ 2.3 km2). Helium fluxes are up to five orders of magnitude higher than the average flux in a stable continental area, pointing to a close link between mud volcanoes and crustal degassing through faults crossing the deep hydrocarbon reservoirs. These data represent a key contribution towards refining global CH4-emission estimates, which indicate mud volcanoes as a significant and unavoidable source of greenhouse gases for the atmosphere.