The structure of the lichen depsidone psoromic acid (1) (4-formyl-3-hydroxy-8-methoxy-1,9-dimethyl-ll-oxo-11H-dibenzo[b,e][l,4]dioxepin-6-carboxylic acid) is confirmed by a combination of spectroscopic, degradative and synthetic experiments. Smiles rearrangements encountered in the degradation of psoromic acid are discussed.