A detailed mechanism for the gas-phase atmospheric reactions of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) in urban and regional atmospheres is comprehensively documented in this report. This can be used in airshed models to determine absolute and relative ozone impacts (reactivities) of the many types of VOCs that can be emitted into the atmosphere, and for other control strategy and research applications. This mechanism, designated SAPRC-99, represents a complete update of the SAPRC-90 mechanism of Carter (1990), and incorporates recent reactivity data from a wide variety of VOCs. The mechanism has assignments for ~400 types of VOCs, and can be used to estimate reactivities for ~550 VOC categories. A condensed version was developed for use in regional models. A unique feature of this mechanism is the use of a computerized system to estimate and generate complete reaction schemes for most non-aromatic hydrocarbons and oxygenates in the presence of NOx, from which condensed mechanisms for the model can be derived. The mechanism was evaluated against the results of approximately 1700 environmental chamber experiments carried out at the University of California at Riverside, including experiments to test ozone reactivity predictions for over 80 types of VOCs. The mechanism was used to update the various ozone reactivity scales developed by Carter (1994a), including the widely used Maximum Incremental Reactivity (MIR) scale. However, the reactivity estimates for many VOC classes are uncertain, which must be taken into account when using these data for regulatory applications. For this reason, uncertainty classifications have been assigned to all VOCs, and upper limit MIRs for VOCs with uncertain mechanisms are presented. OUTLINE1. Introduction2. Base Mechanism3. Generated and Estimated Mechanisms4. Parameterized Mechanisms5. Mechanism Evaluation6. Lumped Mechanism for Airshed Models7. Atmospheric Reactivity Estimates8. References Appendix A. Mechanism Listing and TabulationsAppendix B. Evaluation Tabulations and FiguresAppendix C. Listing of Detailed Model Species and ReactivitiesAppendix D. Estimation of Upper Limit Maximum Incremental Reactivities