Mn­(II) oxidation by free chlorine can be applied to remove Mn­(II) at water treatment plants. This reaction also results in particulate MnO x formation and accumulation in drinking water distribution systems. This study investigated the effect of Fe­(III) and Al­(III) hydrolysis products (mainly precipitates) on Mn­(II) oxidation by free chlorine under drinking water conditions. The results showed that Fe3+ added as FeCl3 and Al­(III) added as polyaluminum chloride (PACl) at tens to hundreds of micrograms per liter dramatically catalyzed Mn­(II) oxidation by free chlorine. Through hydrolytic precipitation at circumneutral pH, Fe3+ and Al13 (the dominant preformed Al species in PACl) generated Fe­(OH)3-like particles and Al13 aggregates, respectively, which initiated heterogeneous Mn­(II) oxidation. Kinetic modeling indicated that, once some MnO x was formed, MnO x and Fe­(OH)3 catalyzed the subsequent Mn­(II) oxidation to an equal extent. The particles (aggregates) formed from Al13 species exhibited a weaker catalytic capacity in comparison to MnO x and Fe­(OH)3 at equivalent molar concentrations. Interestingly, unlike Al13 species in PACl, Al­(III) added as AlCl3 had a negligible influence on Mn­(II) oxidation, even when Al­(OH)3(am) precipitates were formed. The catalytic effects of Fe3+ and Al13 hydrolysis products were confirmed by experiments with natural water and finished water, and the lower Mn­(II) oxidation rate was mainly attributed to organic matter.