The hydration of the hydroxyl OH radical has been investigated by microsolvation modeling and statistical mechanics Monte Carlo simulations. The microsolvation approach was based on density functional theory (DFT) calculations for OH-(H2O)(1-6) and (H2O)(1-7) clusters. The results from microsolvation indicate that the binding enthalpies of the OH radical and water molecule to small water clusters are similar. Monte Carlo simulations predict that the hydration enthalpy of the OH radical, Delta(hyd)H(OH,g), is -39.1 kJ mol(-1). From this value we have estimated that the band gap of liquid water is 6.88 eV, which is in excellent agreement with the result of Coe [J. Chem. Phys. 107, 6023 (1997)]. We have compared the structure of the hydrated OH solution with the structure of pure liquid water. The structural differences between the two systems reflect the strong role played by the OH radical as a proton donor in water. From sequential Monte Carlo/DFT calculations the dipole moment of the OH radical in liquid water is 2.2+/-0.1 D, which is similar to33% above the experimental gas phase value (1.66 D). (C) 2003 American Institute of Physics.