2-Methyl-1-naphthol was found to develop characteristic color with hexoses and pentoses in concentrated sulfuric acid very sensitively, and was used to elucidate the reaction mechanism of the Molisch reaction. The chemical structures of the reaction product of glucose (I) and that of rhamnose (II) were established to be 2-[(4-hydroxy-3-methyl-naphth-1-yl) methyl]-5-[(3-methyl-4-oxo-naphth-1-ylidene) methyl] furan and 5-[(3-methyl-4-oxo-naphth-1-ylidene) methyl]-2-methylfuran, respectively.