Phosphorus ylides combine with carbonyl compds. to give zwitterionic betaines as detectable intermediates of the Wittig reaction, provided that the neg. charge on the O atom was substantially reduced by binding to a Li cation or resonance delocalization into an adjacent carbonyl group. Thus, Bu3P+Me I- was lithiated to give Bu3P+CH2-.LiI, which, with Ph2CO gave Bu3P+CH2CPh2OLi I- which, at room temp., decomposed to give Bu3PO and 1,1-diphenylethylene. Lithiation of Bu3P+CH2CPh2OH I- gave I. The reaction of Ph3P+C-Me2 with Ph2C:C:O, tert-butyl isocyanate, and CO2 gave II which was in equil. with Ph3P+CMe2C(:X)O- (X = CPh2, NCMe3, O). [on SciFinder (R)]