A series of Wittig reactions was calculated at the HF/3-21G* and B3LYP/6-31G* levels to understand the origin of the different pro duct selectivities for different classes of ylides. Both alkylidenetriphenylphosphorane (nonstabilized ylide) and benzylidenetriphenylphosphorane (semistabilized ylide) yielded two types of transition states (TS) with a nearly planar and a puckered structure. The planar TS gave trans oxaphosphetane (OP), whereas the puckered TS led to cis OF. In contrast to previous semiempirical calculations, the present calculations showed that while a planar trans TS is more stable than a puckered cis TS for the semistabilized ylide, a puckered cis TS is more stable for the reaction of the nonstabilized ylide with benzaldehyde. These calculated selectivities agree with experiment. The carbonyl carbon kinetic isotope effects (KIEs) were computed at HF/3-21G* for the reactions of benzaldehyde with butylidenetriphenylphosphorane and with benzylidenetriphenylphosphorane. The reaction of the semistabiiized ylide gave C-13 KIE of 1.051 at 0 degrees C, which is in qualitative agreement with the experimental KIE. In contrast, C-13 KIE for the reaction of the nonstabilized ylide with benzaldehyde was calculated to be 1.039, disagreeing with the experimental isotope effect of unity. This implies that although the product selectivity is reproduced by a pair of the planar trans TS and the puckered cis TS, the latter may not be the true rate-determining TS for the cis-OP formation process for the nonstabilized ylide reaction.