The mechanisms for the substitution of an aqua ligand with F(-) in monomeric Al complexes were studied with density functional theory (DFT). Typical mechanisms are modeled to determine the preferred substitution pathway according to the activation energy barriers. The present computational results are in favor of interchange associative (I(a)) mechanism for the substitution of F(-) into Al(H(2)O)(6)(3+), whereas interchange dissociative (I(d)) mechanism is preferred for the substitution into Al(H(2)O)(5)(OH)(2+), which is in agreement with the previous experimental findings. This implies the mechanistic changeover from I(a) to I(d) induced by the spectator hydroxyl ligand. Like the water-exchange reaction, the substitution rate is accelerated by OH(-) ligand. The difference of the computational and experimental activation enthalpy values is interpreted as the DFT errors in energy and the deviation of transmission coefficient from unity.