Organic compounds are chemically transformed in the troposphere by reaction with photochemically generated oxidants that include hydroxyl radicals, nitrate radicals, and ozone. The reaction rates are a measure of atmospheric persistence and are necessary for developing environmental exposure assessments. Since relatively few experimentally measured rate constants are available, environmental risk/exposure assessors must estimate degradation rates. Rates can be predicted through use of quantitative structure-activity relationships (QSARs). QSAR methods are described for estimating reaction rates with hydroxyl radicals, nitrate radicals, and ozone. QSAR accuracy and limitations are also discussed.