We report here the first measurements of volcanic gas fluxes obtained with a Sun-tracking Fourier transform infrared spectrometer. The experiment was carried out at Masaya volcano in Nicaragua, in April 2000, and yields ratios and emission rates for five gases: H2O, CO2, SO2, HCl, and HF. Traverses beneath the plume were made by road 15 km downwind from the summit, and indicate SO2 and HCl fluxes of 11 and 3 kg s-1, respectively. These values are a good guide to source strengths since we observed no significant change in SO2/HCl ratios (~3 by mass) between the summit and downwind sites, suggesting negligible removal of SO2 or HCl from the gas phase prior to observation. Combining the HCl flux, with the ratios of HF/HCl, CO2/HCl and H2O/HCl determined spectroscopically at the summit, yields fluxes for HF, CO2 and H2O of 0.26, 11 and 160 kg s-1, respectively. The technique heralds considerable promise for volcano monitoring. ¿ 2001 American Geophysical Union