[(Benzyloxy)methyl]dialkylsilyl-substituted 1,3-dithianes show in CI-MS an abundant loss of benzaldehyde from the [M + H]+ quasi-molecular ion. The fragmentation is explained with an intramolecular redox process, where a hydride is proposed to be transferred from the benzyl position to a neighboring thionium ion. This would form a particle that could readily lose benzaldehyde as a neutral fragment. The CI-MS results provide an explanation for the unusual instability of (benzyloxy)methyl-substituted silanes towards acids. In fact, the formation of benzaldehyde was established in the decomposition of a (benzyloxy)methyl-substituted acylsilane in the presence of Lewis or Brønsted acids and ethanethiol. The CI-MS study, therefore, represents a useful method to recognize unusual reactions that are – or might be – important in solution.