The reaction of stabilized Peterson reagents (α-silyl ester enolates) with ketones has been studiedtheoretically and experimentally. Enolate geometry was studied by trapping experiments and NMRspectroscopy and was found to differ markedly with the nature of the base (LiHMDS vs LDA vsKHMDS). The chelating effect of the lithium counterion was found to be critical for the reaction.For the two ketones studied, the combined weight of experimental and computational data assignsgeometrical selectivity to the initial addition transition state, though in general there appears tobe a fine balance between three possible choices for the rate-determining step.