It is generally accepted, but not experimentally proven, that a quantitative prediction of volcanic eruptions is possible from the evaluation of volcanic gas data. By discussing the results of two years of real-time observation of H₂O, CO₂, and SO₂ in volcanic gases from Mount Etna volcano, we unambiguously demonstrate that increasing CO₂/SO₂ ratios can allow detection of the pre-eruptive degassing of rising magmas. Quantitative modeling by the use of a saturation model allows us to relate the pre-eruptive increases of the CO₂/SO₂ ratio to the refilling of Etna's shallow conduits with CO₂-rich deep-reservoir magmas, leading to pressurization and triggering of eruption. The advent of real-time observations of H₂O, CO₂, and SO₂, combined with well-constrained models of degassing, represents a step forward in eruption forecasting.