A series of eleven gas-phase chemical reactions have been examined to assess the dependence of transition state geometries and energy barriers, as well as energy differences between reactants and products, on the a0 B3LYP functional parameter. Throughout the study we have changed the a0 parameter from 0.1 to 0.9 and for the ac and ax parameters we have followed the relationships ax = 1 − a0 and ac = ax. By comparing with the QCISD transition state geometries and energy barriers, our systematic study allows us to identify the influence of the a0 parameter in the reactions studied. In general, B3LYP calculations with the original parameters underestimate energy barriers, this trend being corrected when the a0 parameter increases. Our study also shows that the fraction of Hartree–Fock exchange needed to predict accurate barrier heights differs from the optimal fraction needed to predict thermochemical properties and geometries.