The effects of substituents (H, CH3, CN, OCH3, di-CH3, and di-CN) on the conversion of carbonyl oxides to dioxiranes have been examined in the gas phase and in solution, with B3LYP/6-31G(d,p). The solvent has been modeled with the SCIPCM method. Optimizations in solution have shown that the geometry of carbonyl oxides and the reaction barriers for their conversion to dioxiranes depend on the characteristics of the substituents. The syn isomers of CH3 and OCH3 carbonyl oxides are more stable than their anti counterparts, whereas the conversion of anti substituted carbonyl oxides into dioxiranes is easier for all the substituents. Disubstitution favors the ring-opening reaction of dioxiranes.