The reactivity of the water dimer radical cation toward a series of substrates has been studied in the gas phase using a Fourier transform. ion cyclotron resonance (FT-ICR) mass spectrometer. The water dimer radical cation, H4O2.+, has been prepared at low pressure in the FT-ICR cell through an exchange reaction of the xenon dimer radical cation with water. Under the applied low pressure conditions the water dimer radical cation is shown to react as an electron acceptor, a proton donor, and a reagent which can exchange an OH. moiety. Independent bracketing of both electron transfer and proton transfer processes leads to an energy difference of 1029-1037 kT mol(-1) between the water dimer radical cation and two nonassociated water molecules. This corresponds to an adiabatic ionization energy of 10.81-10.90 eV for the water dimer, (H2O)(2), significantly lower than the 11.21 and 11.1 eV values, obtained from photoionization and photoelectron spectroscopy studies, respectively. The presently obtained results, however, seem to be consistent with results of ab initio calculations.