The key step in the trans-selective modification of the Wittig reaction is the a-lithiation of the lithium bromide coordinated ylide-aldehyde adduct (the so-called \"P-betaine\"). The olefination reactions were carried out for RCHO and [R1CH2PPh3]Br (R = C6H3, Ph, CH2:CMe, iPr, tBu; R1 = Bu, Pr, PhCH2CH2, C5H11) applying PhLi, MeLi, BuLi, sec-BuLi and tert-BuLi as bases. Only PhLi and MeLi yielded (E)-alkenes with good stereoselectivity (>98%), whereas other organolithium bases yielded (E)-(Z)-mixts. Special expt. with (C6D5)3P showed, that phenyllithium acts solely as base, forming a-lithiated betaine, and no P-Ph addn. intermediates were formed. Only phenyllithium effects this deprotonation rapidly and cleanly. Alkyllithiums (in particular, butyl-, sec-butyl-, and tert-butyllithium) react only sluggishly and incompletely, being tied up in very stable mixed aggregates with the lithium alkoxide part of the betaines. The different results of the Wittig reaction are accounted to the low aggregation ability of PhLi and (MeLi)4, which disfavors the formation of mixed product-base aggregates. [on SciFinder (R)]