Vibrationally excited species have been considered to play significant roles in H-2/F-2 reaction systems. In the present study, in order to obtain further understanding of the chain reaction mechanism in the combustion of mixtures containing H-2 and F-2, burning velocities for H-2/F-2/O-2/N-2 flames were measured and compared to that obtained from kinetic simulations using a detailed kinetic model, which involves the vibrationally excited species, HF(v) and H-2(v), and the chain-branching reactions, HF(v > 2) + F-2 = HF + F + F (RI) and H-2(v = I) + F-2 = HF + H + F (R2). The results indicated that reaction RI is not responsible for chain branching, whereas reaction R2 plays a dominant role in the chain reaction mechanism. The kinetic model reproduced the experimental burning velocities with the presumed rate constant of k(2) = 6.6 x 10(-10) exp(-59 kJ mol(-1)/RT) cm(3) s(-1) for R2. The suggested chain-branching reaction was also investigated by quantum chemical calculations at the MRCI-F12+CV+Q/cc-pCVQZ-F12 level of theory.