The initial concentrations of S (1,600 ppm) and Cl (1,100 ppm) in the high-Al2O3 basaltic magma from the 1974 eruptions of Fuego Volcano, Guatemala, were inferred from trapped glass inclusions in phenocrysts. During the explosive eruptions, as much as 33% of the S and 17% of the Cl fell quickly back to Earth as acid aerosol particles absorbed on the ash. An additional 5% of the S and 20% of the Cl was trapped in the silicate ash. The remaining S and Cl was released to the atmosphere. By estimating the volume of ash and applying the above values for S and Cl, 2.2 × 1011 g of S and 1.6 × 1011 g of Cl were calculated to be the atmospheric contribution of the 1974 Fuego eruption. These figures are minimum values because an undetermined amount of intrusive magma may have contributed volatiles to the eruption. Airborne in-the-plume measurements, together with the existing approaches, are the best way to eliminate this uncertainty. The absolute concentrations of scavenged elements on ash are seen to be a function of plume flux and particle trajectory, both of which vary greatly during an eruption. Intense pyroclastic activity produces higher S/Cl ratios in the coating acquired by the ash particles; this implies that there are higher S/Cl ratios in volcanic gas during more explosive phases of an eruption.