Collisional reactions of size- and energy-selected unprotonated water clusters (H2O)(n)(+) (n = 2 and 3) in a single-collision with D2O were investigated at very low collision energy ranging from 0.05 to 2.0 eV using guided-ion beam tandem mass spectrometry. First, in the unimolecular and collision-induced dissociations of (H2O)(2)(+) and (H2O)(3)(+), only the H2O+ fragment, but not the H3O+ fragment, was observed. These results suggest that the structure of (H2O)(2+) is primarily hemibonded '(H2O...OH2)' rather than proton-transferred '(H3O+...OH)'. Second, the measured incorporation cross-section of (H2O)(2)(+) Was found to be smaller than that of H+(H2O)(2). From the dipole moment obtained by ab initio calculation. we found that the small values of the incorporation cross-section for (H2O)(2)(+) were caused by the small value, 0.0 debye, of the dipole moment of the cluster. (C) 2003 Elsevier B.V. All rights reserved.