We investigated the geochemistry of the fumaroles at the summit area of Mt. Etna, including sulfur speciation and the content of acidic gases. The carbon-isotope composition of the Etnean plume was also measured in order to compare it to that of fumaroles. Two types of fumaroles were identified: (i) low-temperature fumaroles, which are dominated by CO2 with minor amounts of SO2 and H2S, and negligible chlorine contents, and (ii) high-temperature fumaroles, which are strongly air-contaminated and characterized by appreciable amounts of volcanogenic carbon, sulfur, and chlorine. As recognized by Martelli et al. (2008), both groups of fumaroles are fed by the degassing of an underlying magma; nevertheless, compositional data clearly show that secondary processes affect the composition of the fluids once they leave the magma body. Here a model of cooling and condensation of fluids is proposed to explore such postmagmatic processes. The model, which uses Etnean plume geochemistry as starting composition of fluids exsolved from magma, shows that SO2 and H2S control the redox conditions of the gas mixture during the cooling, until the reactions involving CO/CO2 and H2/H2O ratios are fully quenched at temperatures around 350–450 °C. The dissolution of gases in water, subsequent to condensation, must occur at thermobaric conditions over 50 bar and 260 °C, which allows (a) total removal of HCl, (b) partial removal of sulfur species while preserving the SO2/H2S ratio, and (c) the C/S ratio to increase by almost 10-fold relative to that in the plume. The observed CH4/CO2 ratios are higher than those calculated for the Etnean magmatic gas, and hence they provide evidence of modest contributions from peripheral hydrothermal fluids during the migration of magmatic gases toward the surface in both low- and high-temperature fumaroles. Due to the peculiar thermodynamic conditions, the model predicts that carbon isotopes do not experience any postmagmatic fractionation, and hence the isotopic composition of the fumaroles is representative of magmatic carbon. Measurements of the carbon-isotope composition of the plume corroborate these findings.