Coumaric acids which ordinarily form hydroxystyrenes on heating yield the corresponding coumarins when heated in the presence of mercuric oxide or chloride. The scope of the use of concentrated sulphuric acid and alcohol saturated with hydrogen chloride for producingtrans tocis inversion has been examined. The former is useful for nitrocoumaric acid whereas the latter is suitable for coumaric and methylcoumaric acids. Mercuric chloride in neutral aqueous solution has been found to be very satisfactory for both types of acids.