•HCO3–/CO32– species leads to changed pH enhancing PMS mediated transformation.•PMS mediated processes are generally inhibited by I– due PMS consumption.•Cl– influences the observed transformation likely due to HOCl generation and reactions.•Br–/PMS induces varied degradation kinetics and mechanisms compared to PMS alone.•HA/phenol/pyridine/sorbate has minimal influence on PMS induced transformation. The rapid and selective peroxymonosulfate (PMS) induced transformation of H1-antihistamines cetirizine (CET) and diphenhydramine (DPH) can be influenced by the presence of common organic and inorganic water constituents. Presence of HCO3– and/or CO32–, which often exhibit powerful inhibition on the advanced oxidation processes (AOPs), can enhance the PMS mediated transformation of CET/DPH. The observed promotion is demonstrated by the changed solution pH through detailed kinetic studies. The impact of halide ions is remarkable, with I– inhibiting the process through consumption of PMS, while Cl– increases slightly the transformation kinetics through the formation and subsequent reactions of HOCl. The CET/DPH degradation in the Br–/PMS system is influenced by the generation of reactive species such as HOBr which leads to different reaction pathways as compared to PMS alone. The results demonstrated the performance of PMS can be tailored through varying the experimental parameters. In addition, the presence of model organic constituents found in water, e.g., humic acid, phenol, pyridine or sorbate, has a minimal effect on the PMS mediated oxidation processes, highlighting the strong application potential of PMS in water treatment. Display omitted