Be2+ (aq) hydrolysis can to lead to the formation of multi-beryllium clusters, but the thermodynamics of this process has not been resolved theoretically. We study the hydration state of an isolated Be2+ ion using both the quasi-chemical theory of solutions and ab initio molecular dynamics. These studies confirm that Be2+ (aq) is tetra-hydrated. The quasi- chemical approach is then applied to then the deprotonation of Be(H2O)(4)(2+)to give BeOH(H2O)(3)(+). The calculated pK(a) of 3.8 is in good agreement with the experimentally suggested value around 3.5. The calculated energetics for the formation of [Be.OH.Be](3+) are then obtained in fair agreement with experiments. (C) 2003 Elsevier Science B.V. All rights reserved.