The mechanistic importance of HMPA and proton donors (methanol, 2-methyl-2-propanol, and 2,2,2-trifluoroethanol) on SmI2-initiated 5-exo-trig ketyl-olefin cyclizations has been examined using stopped-flow spectrophotometric studies. In the presence of HMPA, the rate order of proton donors was zero and product studies showed that they had no impact on the diastereoselectivity of the reaction. Conversely, reactions were first-order in HMPA, and the additive displayed saturation kinetics at high concentrations. These results were consistent with HMPA being involved in a rate-limiting step before cyclization, where coordination of the intermediate ketyl to the sterically congested Sm(III)HMPA both stabilizes the intermediate and inhibits cyclization. Liberation of the contact ion pair through displacement by an equivalent of HMPA provides a solvent-separated ion pair releasing the steric constraint to ketyl-olefin cyclization. The mechanism derived from rate studies shows that HMPA is important not only in increasing the reduction potential of Sm(II) but also in enhancing the inherent reactivity of the radical anion intermediate formed after electron transfer through conversion of a sterically congested contact ion pair to a solvent-separated ion pair. The mechanistic complexity of the SmI2-HMPA-initiated ketyl-olefin cyclization is driven by the high affinity of HMPA for Sm(III), and these results suggest that simple empirical models describing the role of HMPA in more complex systems are likely to be fraught with a high degree of uncertainty. Copyright © 2008 American Chemical Society.