The reactivity of eight purified depsides obtained from six european lichens and that display as 2-oxoalkyl chain in ortho-position of the ester bond was explored. These depsides were found to lead to 1H-Isochromen-1-ones, which exhibit a distinctive blue fluorescence at 365 nm, in the presence of a 10% aqueous solution of KOH. A mechanistic explanation, involving the formation of an enolate intermediate and intramolecular transesterification, was proposed and validated by DFT. By exploiting this fluorescent phenomenon, we conceived a chemical probe (the KUV probe) that is useful for lichen determination, as exemplified on a selection of European Porpidia species.