The effects of the hydroxy group on the conversion of carbonyl oxide to dioxirane 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. All the possible orientations of the hydroxy group (syn-in, syn-out, anti-in, anti-out) on carbonyl oxide and dioxirane (in, out) have been considered. The Si (syn-in) conformer of hydroxy carbonyl oxide was not located as a stationary structure, instead peroxyformic acid is formed. The isomerization barrier for So is found to be twice as large as the one for the Ao and Ai in vacuum and in a polar media (ϵ=78.5). Single point calculations with B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) mimic the same trend.