A theoretical study has been conducted to clarify the discrepancy between ab initio predictions of the entrance channel barrier in the F + H2 reaction and the experimentally observed thresholds for product formation. New results using two different models are in better agreement with the experimental excergicity and predict a zero-point energy corrected barrier of 2.0 kcal mol-1. Estimates of the resulting reaction cross section and thermal activation energy suggest that this value differs from experiment by about 0.5 kcal mol-1. The theoretically predicted isotope effect on the thermal activation energy is larger than either of the recently suggested experimental values, which, however, are subject to large uncertainties. © 1985.