The mobility of Al and Fe in acidic sandy forest soils is greatly influenced by interactions with dissolved organic matter (DOM). We determined the distribution of Al, Fe(II) and Fe(III) over dissolved ‘free’ metal, dissolved metal–DOM complexes and metal–DOM precipitates as influenced by solution pH, redox potential and metal/organic carbon (M/C) ratios. For this, we used water extracts of the Oh horizon of a Fimic Anthrosol brought at pH=3.5, 4.0 and 4.5, and added increasing amounts of Fe(II), Fe(III) and Al. For all three metals, soluble metal–DOM complexes were dominant at low M/C ratios (<0.03). At higher M/C ratios, a strong pH effect was observed for Al: at pH=3.5, dissolved ‘free’ Al was the dominant species, while at pH=4.5, insoluble Al–DOM complexes dominated. For both Fe species, the pH effect was much less pronounced. For Fe(III) at higher M/C ratios, insoluble complexes were the dominant species at all pH values. For Fe(II), very little precipitation was observed and while free metal in solution gained in importance at higher M/C ratios, soluble complexes remained equally important. The differences between Fe(II) and Fe(III) signify the influence of the redox potential of the soil solution. We propose that in horizons where negatively charged sorption sites dominate, cation bridging causes soluble metal–DOM complexation to lead to immobilization of DOM and bound metals, while in horizons where positively charged sites dominate, soluble metal–DOM complexation prevents immobilization by adsorption. Combined with our results and the pH and M/C gradients found in acidic forest soils, this would mean mobilization of Al, Fe and DOM in O, A and E horizons and immobilization in B horizons, as is generally observed in podzols.