When investigating the toxicity of individual particle sources, it is important to consider the contribution of both primary and secondary particles. In this article, we present the design of a new photochemical chamber that can be used to form secondary sulfuric acid particles from diluted coal-fired power plant emissions. The chamber is a relatively small, well-mixed flow reactor that can fit in a mobile reaction laboratory. It produces high concentrations of hydroxyl radical (OH) from the photolysis of ozone (O3) in the presence of water vapor. Two chambers were built and tested. A pilot chamber was tested in the laboratory, using mixtures of NO and SO2 in air, at concentrations that are approximately 100 times lower than those in power plant stack emissions. This chamber was able to oxidize about 20% of the SO2, thereby producing 1350 microg m(-3) of H2SO4 particles. Further tests showed that increasing O3 concentrations and residence time increased the H2SO4 production. A field chamber was built subsequently and used in a toxicological study. Diluted coal-fired power plant emissions were introduced in the chamber. Over 19 days of exposure, the chamber, on average, converted 17% of the supplied SO2 emissions and produced an average of 350 microg m(-3) of H2SO4 particles. Particle losses were determined for the pilot chamber, using artificial particles whose size ranged from 50 to 1000 nm. The determined losses ranged from 21 to 42%, with no trend between the amount of particle loss and particle size. Losses for the field chamber, estimated using model calculations, were found to be similar to those of the pilot chamber.