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The rate of neutralization of sulfuric acid aerosol by ammonia gas has been measured in a laboratory flow reactor for particle diameters between 0.3 and 1.4 micrometers and for relative humidities between 8 and 80%. The rates were between 21 and 70% of the rates expected if diffusion of NH/sub 3/ to the aerosol droplets was the rate-determining factor, and the reaction coefficient, which is the fraction of gas-particle collisions resulting in a chemical reaction, increased with increasing particle size. A simple model of H/sub 2/SO/sub 4/ aerosol generation and neutralization in the atmosphere showed that sulfate aerosol more acidic than NH/sub 4/HSO/sub 4/ should only be present during periods of rapid oxidation of the precursor SO/sub 2/ and high ratios of (SO/sub 2/) to (NH/sub 3/) or when the equilibrium vapor pressure of NH/sub 3/ over the partially neutralized H/sub 2/SO/sub 4/ droplet exceeds the ambient NH/sub 3/ partial pressure.