Abstract: The structure of -chloro--oximino-4-hydroxyacetophenone (COHAP) can be fully characterized by electron ionization and desorption chemical ionization mass spectrometry through a probe introduction of the sample or by the flow injection of particle beam liquid chromatography/mass spectrometry with tetrahydrofuran/hexane elution. With reversed-phase liquid chromatography, or when a normal-phase diol or silica column was employed with nonaqueous mobile phase for the separation of COHAP from its reaction mixture, however, the compound was detected as its dehydrochlorinated product by particle beam liquid chromatography/mass spectrometry. Our studies show that the liquid chromatographic peak which is regarded as COHAP is actually the p-hydroxybenzoyl hydroxamic acid and p-hydroxybenzoyl nitrile oxide under reversed-phase and normal-phase liquid chromatographic conditions, respectively.