Advanced oxidation of a mixture of two benzalkonium chlorides (BACs), benzyl dimethyl dodecyl ammonium chloride (BDDA) and benzyl dimethyl tetradecyl ammonium chloride (BDTA), using O-3 and H2O2 at elevated pH was performed in Milli-Q water and in a municipal wastewater influent. The potential for O-3/H2O2 advanced oxidation process (AOP) to reduce the toxicity of BACs was evaluated. Experimental conditions were optimized using a central composite design (CCD). About 1.28 g H-1 O-3 and 200 mg L-1 of H2O2 at pH 11 degraded 90% of the initial BACs (BDDA:BDTA at 50:25 mg L-1) within 30 min. Formation of 25 intermediates and six degradation pathways for the hydrophilic (benzyl and ammonium moiety) and hydrophobic (alkyl chain) constituents of BACs were proposed. Despite the formation of various intermediates, toxicity of BACs in the treated water, assessed using two fresh water algae species (Chlorella vulgaris and Chlamydomonas reinhardtii), was completely removed by O-3/H2O2 oxidation. Based on the ratio of the 5-day biological oxygen demand (BOD5) to chemical oxygen demand (COD), AOP improved the biodegradability of BACs. (C) 2017 Elsevier B.V. All rights reserved.