Dinitrophenylhydrazone derivatives of volatile aldehydes and ketones were investigated by high-performance liquid chromatography (HPLC) and by gas chromatography-mass spectrometry (GC-MS). HPLC separations were slightly better than those obtained by existing procedures, but high-resolution GC is the method of choice. The equilibrium of the syn-anti isomerization of the compounds is influenced by the nature of the solvent, the deactivation of the column and the injection and column temperatures. The GC profile can therefore be controlled by careful selection of the experimental conditions. It is shown that this phenomenon, in addition to the occurrence of characteristic values of the difference in the retention indices of syn and anti isomers for a given compound, can facilitate the identification. Chemical ionization (methane) mass spectra of the compounds were found to contain more information than conventional electron impact spectra. Characteristic fragment ions are tabulated.