An experimental study of the reaction rate between monodisperse sulfuric acid aerosols and ammonia gas is described. Reactions took place in a laminar flow reactor at 24 °C and 6% relative humidity, and reaction products were sampled from the core of the flow so that reaction times were well defined. For the data reported here, the reaction time was 5.0 ± 0.5 s, ammonia concentrations ranged from 13 to 63 ppb, and particle sizes ranged from 0.03 to 0.2 pm. The extent of reaction was determined by comparing the hygroscopic and deliquescent properties of the product aerosols with known properties of aerosols consisting of internal mixtures of sulfuric acid and ammonium sulfate. It was found that the average fraction of ammonia-aerosol collisions that resulted in chemical reaction during neutralization decreased from 0.40 ± 0.10 for 0.058-ì particles to 0.18 ± 0.03 for 0.10-ì particles. Differential mobility analyzers were used for generating the monodisperse aerosols and also for measuring the hygroscopic and deliquescent properties of the product aerosols.