Ylides are considered as intermediates of the well-known Stevens rearrangement. To investigate the mechanism of this rearrangement, theoretical calculations for the rearrangements of monosubstituted ylides, ZH(2)MCH(2) --> H(2)MCH(2)Z (Z = H, CH3, CH=CH2, SiH3, and GeH3; M = P and As), were performed with the Moller-Plesset perturbation theory up to the fourth order. The IRC calculations at the RMP2 level show that the reaction mechanism is strongly dependent on the migrating group. While the methyl and vinyl migrations are antarafacial with large activation energies of 37-47 kcal/mol, the IRC of the SiH3 migration along with that of GeH3 migration displays that it is a suprafacial process. While the migration of the groups with the heavier elements of Si and Ge utilizes their hypervalency to have a small activation energy of 13 kcal/mol, the transition states for the migrations of the methyl and vinyl groups as well as the hydrogen atom are higher in energy. The RMP2 transition state for the methyl migration is less stable than the radical dissociation limit, H2MCH2 + Z, the radical dissociation-recombination path being more favorable, and it may not exist as real. On the other hand, the transition state for the vinyl migration as well as that for the hydrogen migration is slightly more stable than the dissociation limit. If the entropy effect is properly taken into account, the radical dissociation-recombination path would be more favorable than the concerted mechanism for these migrations as well.