Reaction with the hydroxyl radical (HO center dot) is the primary removal mechanism for organic compounds in the atmosphere, and an important process in combustion. Molecules with unsaturated carbon sites are thought to react with HO center dot via a rapid addition mechanism, with little or no barrier; this results in short lifetimes relative to the saturated alkanes, which undergo slower abstraction reactions. Computational chemistry and reaction rate theory are used in this study to investigate the s-triazine + HO center dot reaction. We report that HO center dot addition at a carbon ring site proceeds with the largest known barrier for addition to an unsaturated carbon (9.8 kcal mol(-1) at the G3X level of theory). Abstraction of a hydrogen atom in s-triazine by HO center dot, forming the s-triazinyl radical + H2O, proceeds with a barrier of only 3.3 kcal mol(-1), and this process dominates over HO center dot addition. Our results are in contrast to those for the analogous reactions in benzene, where the abstraction reaction to phenyl + H2O is slower than the HO center dot addition, which forms a radical adduct that can further react with O-2 or dissociate to phenol + H-center dot. The lifetime of s-triazine toward the hydroxyl radical in the troposphere is estimated at 6.4 years, potentially making it a long-lived pollutant. The aromatic s-triazine (1,3,5-triazine) molecule is a structural feature in herbicides such as atrazine and is a decomposition product of the common energetic material cyclotrimethylenetrinitramine (RDX). While the abstraction reaction dominates for the parent s-triazine, the addition mechanism may be of importance in the atmospheric degradation of substituted triazines, like atrazine, where ring H atoms are not available for abstraction. The high-barrier addition mechanism forms an activated hydroxy-triazinyl adduct which predominantly dissociates to 2-hydroxy-1,3,5-triazine (OST) + H-center dot. This OST species is a known intermediate of RDX decomposition. Results are also presented for isomerization of the less-stable 1,3,5-triazine-N-oxide OST species (which may form via unimolecular pathways in the liquid-phase decomposition of RDX) to 2-hydroxy-1,3,5-triazine. A reaction mechanism is proposed for further oxidation of the s-triazinyl radical, where an OST isomer is also a potential product.