Structures of hydrated singly positive charged calcium-water clusters Ca+(H2O), and their hydrogen-eliminated products (CaOH)(+)(H2O)(n-1) were optimized using the ab initio molecular orbital methods and are compared with cationic magnesium-water clusters which have been investigated previously. For n greater than or equal to 2, the structures of Ca+(H2O)(n) are different from those of Mg+(H2O)(n). In the Mg+(H2O)(n) clusters, a pyramidical Mg+(H2O)(3) forms the first shell. In contrast, a quasi-square-planar Ca+(H2O)(4) is the first shell. The structures of (CaOH)(+)(H2O)(n-1) are also different from structures (MgOH)(+)(H2O)(n-1). The structural difference is attributed to the participation of the d orbitals of Ca atom in the bonding. Despite these structural differences, the core molecular ion (CaOH)+ in the hydrogen-eliminated products (CaOH)(+)(H2O)(n-1) is very similar to the corresponding core ion(MgOH)(+). Both ions, CaOH+ and MgOH+, are strongly polarized to Ca2+O-H and Mg2+O-H. Consequently, the hydration energies of the (CaOH)(+)(H2O)(n) are much larger than those of the corresponding Ca+(H2O)(n). The internal energy change of the hydrogen-elimination reactions of the Ca+(H2O)(n) is positive for n = 1-4 but becomes negative for n greater than or equal to 5, which is consistent with the product switch in the time-of-flight mass spectrum reported by Fuke’s group. The equilibrium constants of the hydrogen elimination reaction are also consistent with the experimental observed isotope effects and the determined metal dependencies.