Primarily trans-olefination has been achieved by rapid equilibration of erythro- and threo-betaines. The betaine ylides (I) are formed by the treatment of betaine-LiX adduct with PhLi or n-BuLi in 1:1 ether-tetrahydrofuran at -30 Deg. The equil. between the diastereoisomeric betaine ylides lies far to the threo-form, so that protonation gives mainly threo-betaine. Subsequent treatment with tert- BuOK gives almost pure trans-olefin. The yields of the trans- olefins were from 55-70% and the proportion of the trans-olefin in the mixt. was 89-99%. [on SciFinder (R)]