The action of alkali on a number of coumarins with methyl groups in the 4-position has been studied in detail. Each of them yields readily only one acid which, though fairly stable, has the property of undergoing change into the corresponding coumarin under the action of heat and dehydrating agents. The acids, however, cannot be converted into stabler isomers by methods which are definitely effective in convertingcis intotrans acids. The methyl ethers of the acids are easily obtained by methylating the acids or the original pyrones in the presence of alkali in an aqueous alcoholic medium. Their properties are those of ethers of thetrans acids since they cannot be converted into isomeric compounds by methods which are definitely successful in convertingcis intotrans ethers. It is therefore concluded that the acids and their ethers derived from 4-methylcoumarins aretrans compounds. The comparatively ready formation of these acids and their ready conversion again into the coumarins are explained on the basis of the tautomeric mechanism which renders the geometrical inversion very facile.