The kinetics and mechanisms of the reactions of aluminium(III) with pentane-2,4dione (Hpd), 1,1,1-trifluoro pentane-2,4-dione (Htfpd) and heptane-3,5-dione (Hhptd) have been investigated in aqueous solution at and ionic strength sodium per-Ϫ3 25ЊC 0.5 mol dm chlorate. The kinetic data are consistent with a mechanism in which aluminium(III) reacts with the -diketones by two pathways, one of which is acid independent while the second exhibits a second-order inverse-acid dependence. The acid-independent pathway is ascribed to a mechanism in which reacts with the enol tautomers of Hpd, Htfpd, and Hhptd 3ϩ [Al(H O) ] 2 6 with rate constants of 0.79(Ϯ0.21), and Ϫ2 Ϫ3 3 Ϫ1 Ϫ1 1.7(Ϯ1.3) ϫ 10 , 7.5(Ϯ1.6) ϫ 10 dm mol s , respectively. The inverse acid pathway is consistent with a mechanism in which reacts with the enolate ions of Hpd, Htfpd, and Hhptd with rate constants 2ϩ [Al(H O) (OH)] 2 5 of and respectively. 6 3 7 3 Ϫ1 Ϫ1 4.32(Ϯ0.18) ϫ 10 , 5.84(Ϯ0.24) ϫ 10 , 1.67(Ϯ0.05) ϫ 10 dm mol s , An alternative formulation involves a pathway in which [Al(H 2 O) 4 (OH) 2 ] ϩ reacts with the protonated enol tautomers of the ligands. This gives rate constants of 4 2.79(Ϯ0.12) ϫ 10 , and for reaction with Hpd, Htfpd, and 5 33 Ϫ1 Ϫ1 3.86(Ϯ0.16) ϫ 10 , 8.98(Ϯ0.25) ϫ 10 dm mol s Hhptd, respectively. Consideration of the kinetic data reported here together with data from the literature, suggest that reacts by an associative or associative-inter-2ϩ [Al(H O) (OH)] 2 5