Low temp. 31P NMR indicated that the reaction of Ph3P+C-RR1 (I, R = R1 = H, Me) with R2COR3 (R2 = R3 = H; R2 = Ph, R3 = H) under salt free conditions gave the corresponding oxaphosphetane (II); in the presence of Li salts, esp. in Et2O, Ph3P+CRR1CR2R3OLi X- (III) were formed. II (R = Li, R1 = Me, R2 = H, R3 = Ph), formed from II (R = R2 = H, R1 = Me, R3 = Ph) and alkyllithium reagents, rearranged slowly at -78 Deg and rapidly at -20 Deg to give Ph3P+C-MeCH(OLi)Ph. The reaction of I with benzaldehydes had r 1.1 which indicated that the transition states were highly polar and that the ylide acted as a nucleophile. The intermediacy of III in the Wittig reaction and in the formation of the oxaphosphetane was discussed. [on SciFinder (R)]