The oxidation mechanism of aldehydes with permanganate was studied in acidic and alkaline media on acetaldehyde, propionaldehyde, pivalaldehyde (2,2Ј-dimethylpropanal) and chloral substrates. On addition of water to acetaldehyde dissolved in organic solvents the rate increased, and therefore it may be stated that the hydrate form is more reactive than the aldehyde form. Acid-catalysed nucleophilic addition of permanganate is suggested. In alkaline medium a mechanism based on electron abstraction from the alkoxy anion of the hydrate is proposed. Deprotonation constants of hydrate could be calculated..