Abstract. The chemical speciation of aluminum was examined in surface water samples from Sphagnum peatlands in north-central Minnesota, from peatlands along the Canadian east coast, and from bogs in the Pennine Mountain area of England. In highly organic ([DOC] x 50 mg L-'), low pH waters, 80-90% of total dissolved Al was complexed with organic matter (OM), while in waters with low DOC ([DOC] x 5 mg L-') 5446% of total dissolved A1 existed as Al+' or other inorganic Al species. Batch titrations of OM with Al revealed a high Al binding capacity, 1.4-2.8 pmol (mg DOC)-', that generally was unsaturated with Al. Titrations of OM with A1 in conjunction with a continuous distribution model were used to determine Al-OM conditional stability constants. Binding capacity (pmol A1 (mg DOC)-') and strength (formation constant) increased from pH 3 to 5 but decreased above pH 5 due to formation of Al-hydroxy species including AI(OH),(s). The high binding capacity of OM in bog waters facilitates metal mobility, especially in low pH (< 5) wetlands where metal solubility is high and OM concentrations are highest. Results showed that the relative degree of organic matter saturation with metal ions was important in modeling A1 speciation in bog waters.