Anhydroerythromycin A is a degradation product of erythromycin formed by a complex internal rearrangement of erythromycin A on exposure to acidic conditions. In acid, erythromycin A forms the enol ether and then undergoes a second internal cyclisation of the C12–OH and C9 enol to afford the ketal, anhydroerythromycin A. Anhydroerythromycin A is an important analytical standard for erythromycin A stability studies.