A gas-phase Carroll rearrangement occurring during electrospray ionization tandem mass spectrometry (ESI-MS/MS) led to the discovery of bridged tricyclic degradation products from cilnidipine and pranidipine under acidic conditions for the first time. The unexpected acid-catalyzed decomposition product of cilnidipine was separated and identified by MS, NMR and X-ray analyses. An acid-catalyzed Carroll rearrangement, a hetero-Diels–Alder reaction and a sigmatropic rearrangement cascade reaction were proposed for the formation of bridged tricyclic degradation products from cilnidipine and pranidipine under acidic conditions. Theoretical calculations supported the proposed reaction mechanism.