Abstract: A kinetic study of the Mannich reaction of 3-thiolcoumarin with diphenylamine was studied. It was found that the mechanism of this reaction is different from the classical mechanism of Mannich reaction. 3-Thiolcoumarin first condenses with form-aldehyde to produce 3-hydroxy-methylthiocoumarin which then reacts with diphenyl-amine with the formation of the a-aminomethylated thioether. The reaction appears to be third order at low concentrations (below 0.2 mol·L~(-1)) and first order at higher concentrations (above 0.35 mol·L~(-1)) of the acetic acid used in the acid catalysis.