Display omitted •The pH affected the destruction of OTC by UV-C only and UV-C/H2O2 AOP.•Second-order rate constant of OTC with HO increased with increasing pH.•The presence of carbonate/bicarbonate enhanced the removal of OTC by UV/H2O2.•The effective role of CO3− for the degradation of OTC was confirmed.•Second-order rate constant of OTC with CO3− was estimated to be 2.9×108M−1s−1. In this study, the influence of pH on the degradation of oxytetracycline (OTC) by UV-C only and UV-C/H2O2 advanced oxidation process (AOP) was investigated. The effect of carbonate and bicarbonate on the photochemical transformation of OTC in UV/H2O2 system was also evaluated. It was found that, by UV irradiation alone, the degradation of OTC was significantly improved with increasing pH; while in UV/H2O2 system, the observed UV fluence based pseudo first-order rate constant of OTC was found to be the highest at pH 5.5 and comparable at neutral and basic pH conditions. The conversion of OTC by UV/H2O2 was not inhibited in the presence of carbonate or bicarbonate. Instead, a slight promotion was observed at high concentrations of carbonate or bicarbonate. The presence of carbonate radical in UV/H2O2/Na2CO3 and UV/H2O2/NaHCO3 systems was demonstrated by adding three different alcohols, i.e., tert-butanol (t-BuOH), methanol (MeOH), and isopropanol (i-PrOH) that barely react with carbonate radical, and by adding 4-chlorophenol (4-CP) which has a high second-order rate constant with carbonate radical. The second-order rate constant of OTC with carbonate radical was subsequently determined. The results indicate that carbonate radical can play an important role on the degradation of organic compounds such as OTC in UV/H2O2 system in the presence of carbonate or bicarbonate that is commonly found in natural environment.