The effect of a single water molecule on the reaction between H2O2 and HO has been investigated by employing MP2 and CCSD(T) theoretical approaches in connection with the aug-cc-PVDZ, aug-cc-PVTZ, and aug-cc-P VQZ basis sets and extrapolation to an infinity basis set. The reaction without water has two elementary reaction paths that differ from each other in the orientation of the hydrogen atom of the hydroxyl radical moiety. Our computed rate constant, at 298 K, is 1.56 x 10(-12) cm(3) molecule(-1) s(-1), in excellent agreement with the suggested value by the NASA/JPL evaluation. The influence of water vapor has been investigated by considering either that H2O2 first forms a complex with water that reacts with hydroxyl radical or that H2O2 reacts with a previously formed H2O center dot OH complex. With the addition of water, the reaction mechanism becomes much more complex, yielding four different reaction paths. Two pathways do not undergo the oxidation reaction but an exchange reaction where there is an interchange between H(2)O2 center dot H2O and H2O center dot OH complexes. The other two pathways oxidize H2O with a computed total rate constant of 4.09 x 10(-12) cm(3) molecule(-1) s(-1) at 298 K, 2.6 times the value of the rate constant of the unassisted reaction. However, the true effect of water vapor requires taking into account the concentration of the prereactive bimolecular complex, namely, H2O2 center dot H2O. With this consideration, water can actually slow down the oxidation of H2O2 by OH between 1840 and 20.5 times in the 240-425 K temperature range. This is an example that demonstrates how water could be a catalyst in an atmospheric reaction in the laboratory but is slow under atmospheric conditions.