Description:    The process of CO 2 flashing through hydrous albite-hedenbergite melt was experimentally examined at a temperature of 1100°C and a pressure of 2 kbar. Carbon dioxide was generated when the melt interacted with calcite, and wollastonite was the predominant synthesized phase. Mafic components were introduced into the hydrous albite melt via the dissolution of natural hedenbergite. Raman spectroscopic data on bubbles of the fluid phase in the quench glass indicate that the CO 2 /H 2 O proportions of the bubbles vary. IR spectroscopic data on the glass prove that the water concentration after CO 2 flashing decreased from 5.5 to approximately 3 wt %. The comparison of the composition of the recrystallized clinopyroxene in contact with melt (with and without CO 2 blowing) indicates that CO 2 oxidizes Fe in the melt. The redox effect of CO 2 is quantified by the empirical clinopyroxene tool for metering oxygen fugacity (oxometer), which was calibrated based on experimental data. The oxygen fugacity in our experiments with CO 2 flashing (estimated by the clinopyroxene oxometer) was NNO + (3.0–3.5). Our estimates with the application of the clinopyroxene oxometer indicate that the maximum oxygen fugacity in the magmatic chambers of Vesuvius and Stromboli volcanoes (which are bubbled with CO 2 ) is also close to NNO + (3.5 ± 0.5). Content Type Journal Article Pages 593-606 DOI 10.1134/S0869591112070053 Authors A. G. Simakin, Institute of Experimental Mineralogy, Russian Academy of Sciences, ul. Institutskaya 4, Chernogolovka, Moscow oblast, 142432 Russia T. P. Salova, Institute of Experimental Mineralogy, Russian Academy of Sciences, ul. Institutskaya 4, Chernogolovka, Moscow oblast, 142432 Russia G. V. Bondarenko, Institute of Experimental Mineralogy, Russian Academy of Sciences, ul. Institutskaya 4, Chernogolovka, Moscow oblast, 142432 Russia Journal Petrology Online ISSN 1556-2085 Print ISSN 0869-5911 Journal Volume Volume 20 Journal Issue Volume 20, Number 7