Products of the reactions of natural coumarins have been studied for the first time: the products of the reaction of osthole with a mixture of concentrated acetic and hydrochloric acid, of oxypeucedanin hydrate with acetone and thionyl chloride, and of decursinol with chromium trioxide. A total of eight new compounds not previously described in the literature has been obtained. The structures of seven of the substances have been established on the basis of their IR, PMR, and mass spectra and chemical reactions.