Few measurements of H2SO4 vapor pressure have been made for sulfuric acid in the temperature and concentration ranges of atmospheric interest because of the very low pressures involved (below 10-4 Pa, or 10-6 torr); no such measurements appear to have been made for sulfuric acid solutions neutralized with ammonia. This work presents measurements of H2SO4 vapor pressure for aqueous sulfuric acid solutions between 55 and 77 wt % H2SO4 (corresponding to about 5-25% relative humidity), ammonium sulfate solids at low humidities, and partially neutralized sulfate solutions with [NH4+]:[SO4=] ratios between 0.13 and 1.0. The vapor pressure data collected over sulfuric acid solutions generally agree with the predictions of Ayers, et al. , although positive deviation was observed for the more dilute solutions. The good agreement between this measurement and previous efforts by absolute techniques suggests that the evaporative coefficient for the H2SO4-H2O system is near unity. H2SO4 vapor pressures over solid ammonium sulfate were measured between 27°C and 60°C; the data were fitted to In p = A/T+ B, with A = -5928 and B = -3.77. The H2SO4 vapor pressures of mixed H2SO4-H2O-(NH4)2SO 4 solutions dropped significantly as the [NH4+]:[SO4=] ratio exceeded 0.5. The results suggest that ammonia could very effectively stabilize molecular clusters of sulfuric acid and water in the atmosphere against evaporation, leading to rates of new particle formation higher than those predicted by binary H2SO4-H2O theory.