||The photochemical reactivities of 20 commercial solvents were measured by experiments using a smog chamber. Each solvent was studied in a simple system comprising pure air, nitrogen dioxide, and the individual test solvent and in a more complex mixture comprising pure air, propylene, toluene, butane, and nitrogen dioxide and the test solvent. To the extent possible, the test solvents were selected to give groups of chemicals that differed systematically in certain structural features so that conclusions could be drawn relating structure and reactivity. Reactivities were evaluated by measuring dosages of nitrogen dioxide, oxidant, ozone, and PAN (peroxyacetyl nitrate); rates of disappearance of nitrogen dioxide, test solvent, propylene, toluene, and butane; eye irritation level; and maximum aerosol level. Some of these measurements varied sufficiently so that they could be used as criteria for ranking the solvents.