The acetyl derivatives of coumaric, 4-methylcoumaric, 5-nitrocoumaric, psoralic and isopsoralic acids have been prepared and their behaviour studied under conditions which bring abouttrans tocis inversion. Under the influence of sunlight, the first three acids produce the corresponding coumarins on prolonged exposure. When heated to above their respective melting points all the acetyl compounds undergo inversion to an extent of 60–80%. Along with the pyrones, highly polymerised resinous compounds are also produced on heating. Treatment with equimolecular amounts of mercuric chloride in aqueous or aqueous alcoholic medium brings about inversion in all the cases with almost theoretical yields.