Vicinal syn dihydroxylation of alkenes is achieved by treating alkenes with potassium permanganate (KMnO₄) in an alkaline medium or by using osmium tetroxide (OsO₄) as a reagent, followed by a reducing agent such as H2S or NaHSO₃. Both methods add a hydroxyl group (OH) to each carbon of the double bond, yielding a glycol in which the two OH groups are added to the same side (syn addition) of the former alkene.