The influence of metal ions, such as Fe^2＋, Fe^3＋, Cu^2＋ and Mn^2＋, on 4-CP degrada-tion was investigated in an aqueous pulsed discharge plasma system with or without the addition of a TiO2 photo-catalyst. From an analysis of the pseudo first-order rate constant （kcp） and energy efficiency （G50%） for 4-CP degradation, the experimental results show that the degrada- tion of 4-CP is much enhanced in the presence of ferrous ions at the optimal concentration of 0.2-0.8 mmol/L or 0.2 mmol/L in an aqueous pulsed discharge plasma without or with the TiO2 system, respectively, and the enhancement is ascribed to plasma induced Fenton and photo-Fenton reactions. Meanwhile, the rank of such metal ions for catalytic effect on 4-CP degradation was Fe^2＋〉 Fe^3＋ 〉 Cu^2＋ 〉 Mn^2＋ and Fe^2＋ 〉 Fe^3＋ 〉 Mn^2＋ 〉 Cu^2＋ for the former and the latter systems, respectively, and the reasons behind this were discussed through the analysis of active species, especially hydrogen peroxide.
