The exerogicity of the reaction F+H2O→HF+OH is sufficient to give HF(ν′ ≤ 1); however, arrested relaxation infrared chemiluminescence experiments on this system show emission from HF(ν′ ≤ 3). The higher vibrational levels are populated by the secondary reaction F(2P)+OH(2II)→HF(1∑+) +O(3P). By a combination of SCF-CI calculations and a rotated Morse curve fitting procedure, it is shown that barrier heights on triplet surfaces which correlate reactants and products of the secondary reaction are too high to provide a reaction path. Instead, the reaction proceeds on a singlet surface to produce an HOF complex, followed by rearrangement and a nonadiabatic transition to the triplet surface. An exit-channel barrier results from the surface crossing. The chemiluminescence data are shown to be in accord with this reaction mechanism.