The Barbier allylation of a series of para-substituted benzaldehydes with allylbromide in the presence of Zn, In, Sn, Sb, Bi, and Mg was investigated using competition experiments. In all cases, the slope of the Hammett plots indicated a build-up of negative charge in the selectivity-determining step. For Zn, In, Sn, Sb, and Bi, an inverse secondary kinetic isotope effect was found (k(H)/k(D) = 0.75-0.95), which was compatible with the formation of a discrete organometallic species prior to allylation via a closed six-membered transition state. With Mg, a significantly larger build-up of negative charge along with a small positive secondary kinetic isotope effect (k(H)/k(D) = 1.06) indicated that the selectivity-determining step was the generation of the radical anion of benzaldehyde. The reaction through a six-membered transition state was modeled using density functional theory with the effect of solvent described by a polarized continuum model. The calculated secondary deuterium isotope effects based on this mechanism were found to be in good agreement with experimental values, thus adding further support to this mechanistic scenario.