Most calculations have been performed for sub-cloud acidification of rain. The roles of chemical oxidation kinetics, mass transport rates for gas scavenging, and the raindrop size distribution have been examined. H2O2 (10 ppb) greatly dominates O3 and Mn(+2) in oxidizing dissolved SO2 to form H(-1) and SO4(-2). HNO3 is important for acidification of rain and it also slightly inhibits SO4(-2) formation. Sub-cloud scavenging of H2O2 and HNO3 is mass transport limited, thus causing the acidification of rain to be mass-transported limited. The Marshall-Palmer distribution results in greater predictions of acidity, SO4(-2), and NO3(-1) than the best distribution for raindrops larger than 0.2 mm. Sub-cloud acidification of rain is strongly dependent on: (a) the concentrations of SO2, H2O2, and HNO3, (b) the sub-cloud fall distance, and (c) the raindrop size distribution.