Gas-phase hydrogen (H) abstractions from molecules by free radicals have been studied extensively. They form the simplest class of elementary reactions and also play a key role in atmospheric chemistry and so are the centerpiece of models of reactivity. Despite intense scrutiny, two fundamental mechanistic issues remain unresolved : (1) Do H abstractions proceed directly or indirectly? (2) Do thermodynamic or electronic interactions determine their reaction barrier? The thermoneutral identity reaction, OH + H2O --> H2O + OH, provides an excellent opportunity to answer these questions, Several theoretically predicted H2O-HO complexes raise the possibility of an indirect mechanism, while no thermodynamic forcing influences the reaction barrier. To examine the various reactivity models, the isotopic scrambling reactions (OH)-O-18 + (H2O)-O-16 --> (H2O)-O-18 + (OH)-O-16 and (OD)-O-16 + (H2O)-O-16 --> (H2OD)-O-16 + (OH)-O-16 are studied in a high-pressure flow reactor. The measured rate constants are (2.3 +/- 1.0) x 10(-13) exp[-(2100 +/- 250)/T] cm(3) molecule(-1) s(-1) over the range 300-420 K ((2.2 +/- 1.0) x 10(-16) at 300 K) and (3 +/- 1.0) x 10(-16) cm(3) molecule(-1) s(-1) at 300 K, respectively. The similarity between the room temperature rates indicates a small secondary isotope effect. While the strong temperature dependence reveals that the predicted complexes do not stabilize the isotope exchange transition state sufficiently to bring its energy below the reactants, the small preexponential factor indicates that the complexes pose entropic constraints. Therefore, the reaction mechanism appears to be indirect. This is clarified by tracing the evolution of reagent electronic interactions and geometrical transformations along the reaction path. Activation energies of isotope exchange reactions are used to constrain the thermoneutral intercept for themodynamically based reactivity models. These thermochemical models are shown to be unreliable. However, a correlation between theoretical (ab initio) and experimental reaction barriers does capture gross reactivity trends. These measurements also exclude kinetic fractionation by OH as an important contributor to the isotopic fractionation of water in the earth’s atmosphere.