Abstract The reaction products from the oxidation of prostaglandins with pyridinium dichromate have been identified by direct chemical ionization mass spectrometry of the underivatized compounds after separation by reversed-phase high-performance liquid chromatography and ultraviolet diode-array detection. The thermal influence on the reproducibility of the dehydration patterns of the mass spectra was studied. The main products from the prostaglandins E 1, E 2, F 1α, and F 2α were the corresponding 15-oxo derivatives. Minor amounts of the 9,11,15-trioxoprostaglandin were formed from PGE, while the oxidation of PGF was less selective, yielding additional dioxo derivatives. Addition of water to the reagent reduced the reactivity, but increased the selectivity in favour of the formation of 15-oxo-PGF during the oxidation of PGF.