Abstract. The electron-impact dissociative ionization of pyridine has been investigated using mass spectrometry. Thirty-two well-resolved mass peaks have been identified in the cation mass spectra and assigned to the most likely ionic molecular fragments. The new sixteen ionic fragments' appearance energies have been determined, and sixteen others remeasured. The total cross-section for electron-impact ionization of pyridine has been measured at 100 eV. Thorough analysis indicates that at least three peaks' groups in the mass spectra occur via the shake-off dehydrogenation mechanism, i.e., by sequential hydrogen atoms elimination from the pyridine parent cation or its fragmentation products. The nature of this process is deciphered.

Key words: Pyridine, shake-off dehydrogenation, ThreSpect, cross-section, thresholds, dissociation, ionization, mass spectra, electrons.