AB: Volatile abundances in magma are commonly considered to be inherited from the melting conditions in the mantle and subsequently modified by degassing processes during magma rising. The addition of volatiles of crustal origin during magma transfer toward the surface has been proposed only in a few instances but no quantitative assessment has shown that such a process can affect volatile emissions of active volcanoes to any significant extent. Here, we illustrate the overwhelming effect of carbonate assimilation on CO2 degassing from several Italian volcanic centers. Extensive interactions between magmas and the several-kilometers-thick sedimentary carbonate basement are documented in Central-Southern Italy by abundant high temperature skarn xenoliths occurring in the eruptive products. By means of carbonate assimilation experiments and mass balance calculations, we estimate that the main trends in major element compositions observed Mt. Vesuvius eruptive products of the last 25 ka are consistent with 6-9wt percent of carbonate assimilation at 6-12 km depth. Important assimilation degrees were also deduced for some volcanoes of the Roman Province. Such process introduces several wt percent of CO2 in the magma, which largely exceeds CO2 solubility in molten silicate and significantly contributes to the important CO2 degassing reported for these volcanic areas. We show that both the emission rates and the carbon isotopic compositions of the degassed CO2 are consistent with decarbonation during assimilation. Several other volcanic centers recognized to be emplaced over thick carbonate sedimentary successions are characterized by important CO2 degassing (e.g. Mt. Etna, Volcan Popocatépetl, Merapi, Lascar Volcano, Erebus) and show magmatic skarn xenoliths in their eruptive products as evidence of decarbonation during high temperature magma-limestone interactions. When measured, the carbon isotopic composition of CO2 emitted by the fumaroles of these volcanoes strongly deviates from the typical mantle-derived magmatic signature and probably indicates an important contribution from sedimentary carbonates. This degassing mechanism, largely neglected so far, needs to be considered for interpreting volcanic gas emissions and its contribution to global CO2 emissions from volcanoes into the atmosphere has to be estimated.