Aqueous trivalent aluminum (Al) ions and their oligomers play important roles in diverse areas, such as environmental sciences and medicine. The geometries of octahedral Al(H2O)(6)(3+) and tetrahedral Al(OH)(4)(-) species have been studied extensively. However, structures of intermediate hydrolysis products of the Al(III) ion, such as the penta-coordinated Al(OH)(2+) species, which exists at pH values ranging from 3.0 to 4.3, and their mode of formation have been poorly understood. Here, we present that a trigonal bipyramidal Al(OH)(H2O)(4)(2+) structure is formed in aqueous solution and how this monomeric species dimerizes to dinuclear [(H2O)(4)Al(OH)(2)Al(H2O)(4)](4+) complex in aqueous solution. The Gibbs free energy change calculations indicate that the formation of the dinuclear complex is preferred over the existence of two single trigonal bipyramidal Al(OH)(H2O)(4)(2+) species in aqueous solution. This study captures the solution dynamics and proton transfer in the oligomerization reactions of penta-coordinated Al(OH)(2+) species in aqueous solution.