HOCl is often assumed to represent the active oxidant in solutions of free available chlorine (FAC). We present evidence that Cl(2)O and Cl(2) can play a greater role than HOCl during chlorination of the herbicide dimethenamid. Reaction orders in [FAC] were determined at various solution conditions and ranged from 1.10 +/- 0.13 to 1.78 +/- 0.22, consistent with the concurrent existence of reactions that appear first-order and second-order in [FAC]. Solution pH, [Cl(-)], [FAC], and temperature were systematically varied so that the reactivity and activation parameters of each FAC species could be delineated. Modeling of kinetic data afforded calculation of second-order rate constants (units: M(-1) s(-1)) at 25 degrees C: k(Cl2O) = (1.37 +/- 0.17) x 10(6), k(Cl2) = (1.21 +/- 0.06) x 10(6), and k(HOCl) = 0.18 +/- 0.10. Under conditions typical of drinking water chlorination, Cl(2)O is the predominant chlorinating agent of dimethenamid. To the extent that Cl(2)O represents the active species in reactions with other disinfection byproduct (DBP) precursors, the influence of [FAC] and pH on DBP precursor reaction rates is different than if HOCl were the principal oxidant. Moreover, these findings call into question the validity of apparent rate constants (k(app)) commonly reported in the chlorination literature.