Aluminum (Al)-humus complexes are abundant in humus-rich horizons of nonallophanic Andosols and contribute the unique properties of volcanic ash soils, such as high reactivity with phosphate and fluoride ions and a low bulk density. Nonallophanic Andosols commonly show Al toxicity to plant roots. There have been very few studies concerning contribution of Al-humus complexes to Al toxicity, though it is probable that the complexes are source of toxic Al or even directly injure plant roots. To verify the possibility of toxicity of humus-complexed Al, we extracted humic substances from an A horizon of a nonallophanic Andosol with NaOH solution, and reacted the humic substances and partially neutralized AlCl₃ solution at three pH conditions (pH 4.0, 4.5, and 5.5) to prepare pure Al-humic substance complexes (Complex (4.0), Complex (4.5) and Complex (5.5)). We determined the contents and status of carbon and aluminum in the complexes, and performed a plant culture experiment in the mixture of the complexes and perlite. Aluminum and carbon contents of the complexes reflected the difference of pH at time of synthesis: Molar ratios of Al to C were 0.04, 0.08, and 0.16 for Complex (4.0), Complex (4.5), and Complex (5.5), respectively. The solid state CPMAS ¹³C-NMR spectra of the complexes showed the dominance of aromatic C and carbonyl C. The optical absorption spectra of the NaOH extracts from Al-humic substance complexes possessed similar absorption patterns to A-type humic acid with a high degree of humification. Solid state magic angle spinning ²⁷Al nuclear magnetic resonance analysis showed the existence of Al₁₃ polymer in Complex (4.5) and Complex (5.5), whereas the polymer was not detected in Complex (4.0). It was indicated that all the synthetic Al-humic substance complexes easily and rapidly release monomeric Al into liquid phase by slight changes of pH and ion strength, although the contents of Al and their extent of polymerization are considerably different among the complexes: Relationship between pH and pAl obtained by an equilibrium study showed the Al solubility that was quite different from that of Al(OH)₃, indicating that the solubility was largely controlled by the exchange reaction of H⁺ and Al ions on negative charges of humus. An experiment of Al release rates by a stirred-flow method revealed that monomeric Al was rapidly released from the complexes in the presence of dilute (10^-3 M) acetate buffer solution adjusted to pH 3.5. Results of the plant growth test using the medium of the Al-humic substance complexes and perlite mixture showed that all the roots of burdock and barley were equally injured in the three complex media. Plant roots represented the typical symptom injured by Al toxicity, such as stubby in appearance and brown on the tips.These results indicate that, in soils dominated by Al-humus complexes, Al released from Al-humus complexes, as well as exchangeable Al adsorbed by soil minerals, definitely show toxicity to plant roots.