Equilibrium constants are reported for the aluminum(III) complexes with four ligands that contain the 1,2-dihydroxyaryl group: trans-3-(3',4'-dihydroxyphenyl)propenoic acid (caffeic acid), 3-(3',4'-dihydroxy-cinnamoyl)-1,3,4,5-tetrahydroxycyclohexanecarboxylic acid (chlorogenic acid), 3,4-dihydroxybenzylamine (DHB), and 1,2-dihydroxyanthraquinone-3-sulfonic acid (DASA). The protonation and complexation reactions were studied by potentiometric and spectrophotometric titrations in aqueous solution at 25 degreesC (I = 0.10 M KCl). The complexation reactions were studied for ligand/metal ratios in the range from 1.3 to 5.9. For caffeic and chlorogenic acids (H3L), the experimental data were consistent with the formation of six mononuclear Al-ligand species, each involving catecholate coordination: AlHL+, AlL, Al(OH)L-, AlL23-, Al(OH)L-2(4-), and AlL36-. For DHB (H2L), the data were consistent with the formation of six monomeric and one dimeric Al species: AlHL2+, Al(HL)(2)(+), Al(HL)(3)(0), Al(HL)(2)L-, Al(HL)L-2(2-), AlL33-, and (OH)(2)Al-2(HL)(2)(2+), in which HL has a protonated amino group. On the basis of the DASA (H2L-) titration data, three monomeric and one dimeric Al-ligand species were proposed: AlL0, AlL23-, Al(OH)L-2(4-), and Al-2(OH)(2)L-4(8-), The relative stability of the 1:1 complexes, AIL, was DASA > chlorogenic acid > caffeic acid > DHB. This indicates the progressive electron-withdrawing effects of the ring substituents - SO3- < -CH=CH.CO.O.(C6H10O3).COOH < -CH=CH.COOH < -NH3+.