The Acid Precipitation Act of 1980 called for a comprehensive ten-year program to identify the causes and sources of wet and dry acid deposition, to evaluate their effects, and, on the basis of the findings, to minimize or eliminate their sources where possible. The U.S. Environmental Protection Agency (EPA) is involved with several aspects of the overall interagency National Acid Precipitation Assessment Plan, including field monitoring and quality assurance. One of the major precursors of acid deposition is the gas sulfur dioxide (SO2). The purpose of the project described in this paper was to provide the EPA with standards or predictable test atmospheres of SO2 in air in the parts-per-billion range. Since the amount of calibration gas required was great, it was not feasible to prepare the required mixtures in cylinders, characterize them, and supply them to the EPA. To overcome this limitation, dynamic dilution was chosen as a method of supplying the required mixtures. Gas cylinder mixtures of higher concentrations of SO2 were characterized with respect to concentration and stability over time. These mixtures were used as inputs to the dilution system and blended with large amounts of air to produce SO2 mixtures of known lower concentration. The analysis of the high-concentration cylinder mixtures is described, along with the characterization and evaluation of the components of the dilution system and the dilution system as a whole.