In this study we describe the results of the application of several ab initio methods to the reactions in the gas phase of four unstabilized ylides (H3P=CH2, H2MeP=CH2, HMe2P=CH2, and Me3P=CH2) with formaldehyde to form their respective oxaphosphetanes. At the HF level, H3P=CH2 and H2MeP=CH2 proceeded to the formation of oxaphosphetane. However, at the B3LYP, MP2, and QCISD levels, these ylides react by a nucleophilic attack of the ylidic carbon on the carbonyl group, concomitant with the proton abstraction from the phosphorus atom to form 2-phosphinoethanol. This unusual dependence of the reaction path on the level of theory indicates that H3P=CH2, the most popular ylide for modeling the Wittig reaction, is atypical and raises questions regarding its use as a suitable model for more realistic systems. At all levels of theory (HF, B3LYP, and MP2), the reactions of HMe2P=CH2 and Me3P=CH2 with formaldehyde proceed in a cycloaddition-like fashion to yield oxaphosphetanes. The calculated barriers for these processes varied considerably with the level of correlation and the basis sets employed. The geometries of reactants, intermediates, transition states, and products did not change significantly with the level of theory or basis set employed. The use of B3LYP or MP2 calculations with the 6-31G* basis set is a reasonable compromise between computational expense and level of rigor to describe the Wittig reaction. Our results suggest that for alkyl-substituted ylides, HF geometries along the reaction profile resemble the ones obtained with B3YLP and MP2 methods. Therefore, the Wittig reaction can be properly described at the ab initio level using B3YLP or MP2 single-point energies on the respective HF geometries.