The action of mercuric acetate on the methyl ethers of coumarinic and coumaric acids and their 5-nitro derivatives has been described. The first two undergo addition at the double bond as well as mercuration and the compounds have the carboxylic formula. The nitrocoumarinic and coumaric acids undergo addition only and yield the carboxylic form in the cold and the anhydro form on heating. When the addenda are eliminated by the action of dilute hydrochloric acid the change is accompanied by geometrical inversion fromcis totrans. The results so far obtained with cinnamic acid and its derivatives are reviewed.