behaviour of substituted 1,3-oxazolidin-2-ones (1) and 1,3-oxazolin-2-ones (2) on treatment with acid is reported. Electrophylic addition of iminium cations to 1,3-oxazolin-2-onic monomers occurs, yielding the kinetically preferred dimers (3). The elimination pathway to 3 is in direct competition with the formation of the cyclodimerisation products resulting from a thermodynamicallycontrolled process.