Reaction of cis- and trans-PhCH:CHF with RLi (R = Bu, Me, Ph, or PhCH2) gave stereoselectively cis- and trans-PhCH:CHR, with retention of configuration by substitution and PhC.tplbond.CH by elimination reaction. The elimination reaction follows mainly the E2cb mechanism. The substitution reaction is accomplished by an addn.-elimination sequence with extremely short-lived intermediates. These mechanisms are based on isotope effect, B-ligand effect, and F/Cl elemental effect measurements. The enhanced reactivity of vinyl fluorides towards substitution might be caused by repulsive interactions of neighboring nonbonding p-electrons. [on SciFinder (R)]