Reactions of ozone (O-3) with the beta-lactam antibiotics penicillin G (PG) and cephalexin (CP) have previously been found to yield products retaining antibacterial activities. These products are unequivocally identified here as the stereoisomeric (R)-sulfoxides of each parent molecule and characterized by a combination of chemical analysis and an antibacterial activity assay. PG-(R)-sulfoxide, which is similar to 15% as potent as PG itself, is formed in similar to 55% yield, whereas CP-(R)-sulfoxide, which is similar to 83% as active as CP, is formed with a maximum similar to 34% yield. PG-(R)-sulfoxide is recalcitrant toward further oxidation by O-3, but readily transformed by hydroxyl radical (HO center dot) k(HO center dot)(,app)(N) = 7.4 x 10(9) M(-1)s(-1), pH 7), resulting in quantitative elimination of its antibacterial activity. In contrast CP-(R)-sulfoxide is degraded by both O-3 and HO center dot (k"(O3,app) = 2.6 x 10(4) M-1 s(-1) and k"(HO center dot,app) = 7.6 x 10(9) M-1 s(-1), pH 7), leading to quantitative elimination of its antibacterial activity. During ozonation of a secondary municipal wastewater effluent sample (pH 8.1, C-DOC = 4.0 mg/L, [alkalinity] = 3.6 mM as HCO3-) spiked with [PG](0) = 1 mu M, PG-(R)-sulfoxide yields did not exceed 0.15 mu M for O-3 doses up to 100 mu M (4.8 mg/L), but reached 0.47 mu M with 10-mM t-BuOH added as a HO center dot scavenger. In contrast, CP-(R)-sulfoxide yields did not exceed 0.1 mu M for the same wastewater spiked with [CP](0) = 1,mu M in either the presence or absence of t-BuOH, indicating that CP-(R)-sulfoxide transformation is governed primarily by direct reaction with O-3. These findings suggest that, for a given degree of parent compound transformation, PG-(R)-sulfoxide yields would likely be greatest during zonation of wastewaters characterized by low O-3 demands and high HO center dot scavenging rates, whereas CP-(R)-sulfoxide yields would be less matrix-dependent In general, complete deactivation of penicillins during wastewater treatment will likely require higher O-3 exposures than necessary for deactivation of cephalosporins.