In this study, three forms of faujasite zeolite Y, i.e. ammonium, proton and sodium form, were tested as catalyst supports for ferric ions (Fe 3+) in the oxidation of aqueous formaldehyde (FA) in the presence of H2O2 and UVC. The effect of charge-balancing cations on the FA oxidation at acidic and alkaline conditions was investigated using a batch reactor. The results showed that the three tested catalysts behave similarly at pH 3. However, the presence of charge-balancing proton gives the highest conversion of FA (26%) after 120 min at pH 7, which is comparable to the results obtained at pH 3, indicating the confined space in the supercage of Fe/H-Y provides a pH environment for Fenton reaction different to that provided by Fe/NH4-Y and Fe/Na-Y. Turnover frequency given by Fe/H-Y is more than 3 times faster than that given by Fe/NH4-Y or Fe/Na-Y. We proposed the surface proton, which is in close proximity to Fe3+, mitigates the leaching of metal and allows photo-Fenton process similar to what is generally observed at acidic pH condition. No change in solution pH was observed after the addition of all three tested form of zeolite. We concluded that the localized acidic setting given by the proton inside the porous structure of zeolite is able to promote photo-Fenton reaction at alkaline condition in a convenient and effective way. ; Department of Civil and Environmental Engineering