Ab initio SCF calculations have been performed for the model Wittig reaction given in the title. The basis sets used range from STO-3G to double-f plus polarization quality. Full geometry optimization has been performed with the STO-3G and double-f basis sets. The reaction takes place in an essentially concerted way. Starting from PH3CH2 + CH20 an oxaphosphetane ring is formed via a very small energy barrier. This ring system corresponds to a local minimum on the energy hypersurface. An energy barrier of ~25 kcal/mol is found for the dissociation of the oxaphosphetane ring into PH30 and C2H4. The total reaction energy is calculated to be about -45 to -50 kcal/mol.