An unusual mechanism in the reaction, O(P-3) + H -> ClO + H, dominates at hyperthermal collision energies. This mechanism applies to collision geometries in which the H atom in the HCl molecule is oriented toward the reagent O atom. As the Cl - O bond forms, the H atom experiences a strong repulsive force from both the O and Cl atoms. The ClO product scatters forward with respect to the initial velocity of the O atom, and the H atom scatters backward. This mechanism accounts for more than half the reactive trajectories at energies >110 kcal mol(-1), but it does not involve motion near the minimum energy path, which favors an S-N(2)-like reaction mechanism where the H atom is oriented away from the reagent O atom during the collision.