4-Lithio-2-isoxazolines are oxidized to the 4-peroxides or 4-hydroxy derivatives by air or oxygen at -80.degree. C. The concentration of oxygen in the solution is a decisive parameter. When it is bubbled into the lithium salt solution, the 4-hydroperoxide derivatives are mainly formed. However if oxygen is slowly absorbed in the liquid phase, 4-hydroxy derivatives predominate. In the reaction, the hydroperoxide anions has been confirmed to be the precursors of the 4-hydroxy-2-isoxazolines.