Experiments were performed to investigate the photochemical formation of nuclei and aerosols in sulfur dioxide-air mixtures and the photochemical formation of ozone, nuclei, and aerosols in cyclohexene-nitrogen dioxide-air mixtures. The mixtures were irradiated by ultraviolet fluorescent lamps while flowing at a steady speed through a 9.12 m long and 15.2 cm i.d. Pyrex tube. The amounts of ozone, nuclei, and aerosols generated in the gas-air mixtures were measured at various points along the tube by a chemiluminescent analyzer, a condensation nuclei counter, and an aerosol counter, respectively. Photooxidation experiments with sulfur dioxide-air mixtures were performed for five different initial sulfur dioxide concentrations (0.2, 0.3, 0.5, 0.7, and 1 ppm) and for four relative humidities (25, 50, 75, and 100%). The data showed that the maximum number of nuclei produced is directly proportional to the initial sulfur dioxide concentration and to the square of the relative humidity. There were no measurable amounts of aerosols observed during the tests. Experiments with cyclohexene-nitrogen dioxide-air mixtures were performed with the initial cyclohexene concentration ranging from 0.5 to 50 ppm and the initial nitrogen dioxide concentration from 0.6 to 10 ppm.