We report here on the real-time measurement of CO2 and SO2 concentrations in the near-vent volcanic gas plume of Mount Etna, acquired by the use of a field portable gas analyzer during a series of periodic field surveys on the volcano's summit. During the investigated period (September 2004 to September 2005), the plume CO2/SO2 ratio ranged from 1.9 to 10.8, with contrasting composition for Northeast and Voragine crater plumes. Scaling the above CO2/SO2 ratios by UV spectroscopy determined SO2 emission rates, we estimate CO2 emission rates from the volcano in the range 0.9--67.5 kt d-1 (average, 9 kt d-1). About 2 kt of CO2 were emitted daily on average during quiescent passive degassing, whereas CO2 emission rates from Etna's summit were 10--40 times larger during the 2004--2005 effusive event (with a cumulative CO2 release of ~3800 kt during the 6 months of the eruption). Such a syneruptive increase, ascribed to the replenishment of the shallow (<6 km) volcanic plumbing system by CO2-rich (0.25 wt %) more primitive magmas, supports the potential of CO2 output rates as key parameters for volcanic hazard assessment.