Homogeneous stereoisomeric olefins of the general formula I (R1 and R2 = alkyl, R3 = H and(or) alkyl) were prepd. by reacting a phosphorus ylide (general formula Ar3P+:C-HR) and a carbonyl compd. R2COR3 with stoichiometric amt. of a strong base, followed by addn. of a proton donor to convert the intermediate product into an olefin and triarylphosphine oxide. In a sample, the phosphorus ylide was prepd. by reacting Ph3P+EtCl- with PhLi (1 mole) in a mixt. of ether 3 and tetrahydrofuran 1 part at 0 Deg. The phosphorus ylide was reacted with 1 mole BzH, and the resulting betaine type intermediate was then reacted at -20 Deg with 1 mole PhLi. After .apprx.30 min. the mixt. was dild. with 2 moles tert-BuOH and afterthe addn. of 1.5 moles KOBu-tert stirred at 25 Deg for 1.5 hrs. b-Methylstyrene was isolated in 69% yield having trans to cis ratio 99:1. [on SciFinder (R)]