3,4-Dihydro-2-H-pyran and oxalylchloride react, depending on the conditions, to keto esters, a pyran-3-carboxylic acid or derivatives thereof, or to an hitherto unknown bicyclic acetal containing a vinyl chloride moiety. The structure of the latter product has been unambiguously elucidated by single-crystal X-ray structure analysis. A mechanism for its formation is proposed. J. Heterocyclic Chem.