Heterogeneous Fenton-like catalysts have received considerable research attention because they could potentially be attractive for oxidative removal of organic contaminants from tertiary wastewater.However, process design is still hampered by insufficient understanding of the chemical pathways involved, and especially whether oxidation activity stems from heterogeneous surface chemistry or minute concentrations of dissolved metal ions in the homogeneous phase. Using inductively coupled plasma-optical emission spectroscopy (ICP-OES) in combination with pH monitoring and ultravioletvisible spectroscopy (UV-Vis) we have monitored the degradation of 4-chlorophenol (4-CP) over two Fenton-like heterogeneous systems, namely FeOx supported on TiO2 and CuFe2O4. We show conclusively that these systems proceed predominantly through a homogeneous route via dissolved metal ions from the solid phase catalysts. Control experiments with homogeneous Fe 3+ or Cu 2+ systems reveal that even minute concentrations ( M/sub-ppm) of dissolved metal ions leached from the solid phases account for the observed 4-CP degradation rates in the heterogeneous systems. ICP-OES revealed that metal leaching was time-dependent and variable because of pH variations associated with changing acid release rates. Buffering solutions at pH 7.4 suppressed metal leaching (and hence 4-CP degradation) in the FeOx/TiO2 system, but not in others. For example, pH buffering did not entirely suppress metal leaching from CuFe2O4, for which 4-CP degradation was retained through small concentrations of Fe and Cu ions in solution. Our results highlight the importance of careful monitoring of metal content in the aqueous phase, certainly with analytical sensitivity below ppm concentrations of the dissolved metals, and also the crucial influence of time-dependent pH variations on the reaction process. Recyclability of catalysts, pH buffering of solutions or monitoring of metal content in the solid phase by less sensitive analytical methods, e.g., chemical analysis, gravimetry, X-ray fluorescence, or energy dispersive X-ray analysis in electron microscopes, cannot exclude the homogeneous Fenton route in the presence of solid catalysts. 2Keywords CuFe2O4, leaching, homogeneous, heterogeneous, catalysis IntroductionFenton's reaction is an efficient way for treating organic contaminants, e.g. for soil remediation, 1 oil spill clean-up, 2 and industrial wastewater treatment. 3 Through the addition of Fe 2+ , H2O2 and acid to wastewater streams OH radicals ( • OH) can be generated through the oxidation of Fe 2+ (Fenton's reaction, eq. 1).• OH is the active species that degrades organic contaminants. Fenton's reaction proceeds optimally around pH 3, thus wastewater streams need to be acidified. 5 Subsequent neutralisation leads to Fe(OH)3 precipitates as a sludge, requiring an additional separation step. 6 This complexity of the homogeneous Fenton route motivated investigations of heterogeneous Fenton-like systems, seeking to localise the Fenton process at the su...