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We investigated the influences of humic acid on the removal of Al tridecamer ([Al.sub.13]) from a bydroxyahiminum (HyA) solution at various humic acid/Al ratios. The Al species contained in the solution were analyzed by using a liquid-state [sup.27]Al-NMR and an atomic absorption spectrometer and fractionated into three Al species: (i) [Al.sub.13], (ii) Al monomer and dimer ([Al.sub.SYM]), and (iii) other undefined species including aggregated/precipitated Al ([Al.sub.NON]). By the addition of humic acid to the HyA solution, the concentration of [Al.sub.13] was rapidly decreased within 0.007 d (10 min). The decrease in [Al.sub.13] and the increase in [Al.sub.NON] were more pronounced at a higher humic acid/Al ratio. When the molar ratio of humic acid carboxylic groups to Al exceeded 0.8, [Al.sub.13] was undetected from solution within 0.007 d. The formation of [Al.sub.13]-humic acid complexes and the aggregation/precipitation of those complexes were a predominant mechanism in removing aqueous [Al.sub.13] at the early stage of the reaction. Approximately 10 mol of carboxylic groups in humic acid were required to remove 1 moi of [Al.sub.13] from the HyA solution. Aqueous [Al.sub.13] had greater preference in precipitating with humic acid than [Al.sub.SYM]. After 5 to 570 d of aging, the concentration of [Al.sub.13] and [Al.sub.NON] also decreased and increased, respectively, both in the presence and absence of humic acid. In conclusion, aqueous [Al.sub.13] would not exist in soil solution under a high humic acid condition.