Ab initio calculations for the potential barrier height for the symmetric H-atom exchange reaction HO. + H2O --> H2O + (OH)-O-. are reported. A value of 42.2 kJ mol(-1) is found using the QCISD(T)/6-311+G(3df,2p) method. Multireference CISD calculations converge toward a similar value for the barrier provided that a Davidson correction is applied. The effect of quantum mechanical tunneling is investigated. Rate constants calculated by using conventional and small-curvature tunneling-corrected transition state theory with the UMP2/ 6-311G(d,p) transition structure and reaction path are compared for a wide range of temperatures. Tunneling reduces the Arrhenius activation energy, obtained from the temperature dependence of the calculated rate constants, by at least 20 kJ mol(-1) at 300 K. The best theoretical estimate for the Arrhenius activation energy at 300 K is 21.2 kJ mol(-1); the discrepancy between this and the experimental value of 17.6 +/- 2 kJ mol(-1) is likely to be due to neglect of large-curvature tunneling effects. The QCISD(T)/6-311+G(3df,2p) calculated enthalpy of association of HO. + H2O --> HO.... HOH, the hydrogen-bonded precursor complex, is -8.9 kJ mol(-1). The best theoretical estimate for the intrinsic barrier height for the symmetric H-atom exchange HO.... HOH --> HOH ...(OH)-O-. is 25.1 kJ mol(-1).