Although CO2 and H2O account for more than 90 mol% of volcanic gases, the rates at which these gases are emitted from volcanoes are difficult to determine because of their high atmospheric background levels. We report the first precise field measurements of volcanic CO2, and H2O, in addition to HCl, HF, and SO2, in the plume of Masaya volcano, Nicaragua, a basaltic volcano with a record of Plinian activity. The molar ratios for CO2: SO2 (2.3–2.5) and H2O: SO2 (66–69) observed in February–March 1998 and March 1999 show no significant variation over the 12 month period. The molar composition of the gas is similar to other basaltic arc volcanoes in Central America. Emission rates of SO2 from the summit crater, determined by correlation spectroscopy, averaged 21 kg s−1 during the study periods, indicating CO2, H2O, HCl, and HF emission rates of 32–36, 380–420, 7.0–7.8, and 0.86–0.95 kg s−1, respectively. At these rates it takes only a few years to emit the equivalent volatiles associated with Masaya's prehistoric Plinian eruptions.