The low (3.84 kcal mol⁻¹) transition state for the F + H₂O → HF + HO reaction on the ground electronic state potential energy surface is pre-staged by a relatively deep (2.67 kcal mol⁻¹) van der Waals well in the entrance channel. Quasi-classical trajectory studies indicated that this pre-reaction van der Waals complex “guides” trajectories towards the “reactant-like” transition state. Such a stereodynamic effect is shown to enhance the reactivity at low collision energies, resulting in a pronounced peak in the excitation function. A similar enhancement effect was also found in quantum dynamic calculations.