AbstractProtonated 4,5‐diacetoxyphenanthrene and 2,2′‐diacetoxybiphenyl dissociate, owing to the interaction of the two acetoxy groups, by eliminating a molecule of acetic acid. This novel proximity effect, which occurs for a fast atom bombardment‐produced closed‐shell ion, was examined in detail using tandem mass spectrometric methods. This process, which was first observed in the decompositions of acetylated complex natural products such as stentorin and hypericin, may serve as a general process for determining the proximity of hydroxyl groups in polycyclic aromatic