Hydroxamic acid (HA) ligand-exchange kinetics have been investigated at 25 °C over the [H+] range 1.0-0.001 M (I = 1.0 M (HClO4/NaClO4)). A series of five synthetic hydroxamic acids (R1C(O)N(OH)R2) were chosen for study with varying R1and R2substituents as follows: acetohydroxamic acid, R1= CH3, R2= H.; benzohydroxamic acid, R1= C6H5, R2= H.; 4-acyl-N-phenylacetohydroxamic acid, R1= CH3, R2= 4-CH3C(O)C6H4; N-phenylacetohydroxamic acid, R1= CH3, R2= C6H5; 4-methyl-TV-phenylacetohydroxamic acid, R1 = CH3, R2= 4-CH3C6H4. The kinetic results are consistent with a parallel-path reaction scheme: [formula omitted]. No proton ambiguity exists in the kinetic results due to the weak acidity of the hydroxamic acids and the relatively large hydrolysis constant (2H) for Al(H2O)63+. Complexation rate constants are relatively insensitive to entering group {k1varies from 0.15 to 0.23 M-1s-1; k2varies from 2300 to 2600 M-1s-1), and ligand-exchange rates at Al(H2O)5OH2+are 104times faster than at Al(H2O)63+. Hydroxamic acid ligand dissociation rate constants are sensitive to leaving group (k-1varies from 2.3 X 10-2to 9.3 X 10-1M-1s-1; k-2varies from 4.8 X 10-3to 1.4 X 10-1s-1), and variations in In k-1are linearly related to In k-2. These results are discussed within the context of an interchange mechanism, where complex formation energetics are dominated by water exchange at aluminum Results reported here for aluminum(III) are compared with the corresponding ligand-exchange reactions for iron(III) previously reported from our laboratory. © 1987, American Chemical Society. All rights reserved.