Existing studies of the composition of volcanic plumes generally interpret the presence of sulfate aerosol as the result of comparatively slow oxidation of gaseous SO2. We report here new observations from Masaya Volcano, Nicaragua, which demonstrate that sulfate aerosol may also be emitted directly from volcanic vents. Simultaneous aerosol and gaseous S, Cl, and F compounds were collected at the rim of the passively degassing crater in May 2001. Mean concentrations of SO42-, Cl-, and F- within the plume were 83, 1.2, and 0.37 ¿g m-3, respectively (fine aerosol fraction 2.5 ¿m). The aerosols were highly acidic, with estimated pH of <1.0 in the fine aerosols. Sulfate was present mainly in smaller particles, with the fine fraction accounting for ≈80% of the mass. The bulk of the sulfate was emitted directly from the magmatic vent. Acidity in the aerosols derived from the presence of sulfuric acid and, to a lesser extent, hydrofluoric acid, with +>/42-> equivalent values of 0.5--0.8 and 0.3--3 for fine and coarse aerosols, respectively. Gas phase/aerosol phase mass ratios were, on average, 458 (S), 330 (F), and 186 (Cl), with ranges of 95--1178, 37--659, and 43--259, respectively. These observations of highly acidic aerosol emitted directly from crater vents have implications for plume chemistry and environmental and health impacts of volcanic degassing.