Abstract: 10 polychlorinated polycyclic aromatic hydrocarbons (PCAHs) were analyzed by electron bombard ionization mass spectrometry (EI-MS) and MS equipped with atmospheric pressure chemical ionization source (APCI-MS). In the EIMS mode, the C—Cl bonds in all the PCAHs are liable to break, and the abundance of the peaks which lose even Cl atoms are stronger than that of peaks which lose odd Cl atoms. In the APCI-MS mode, the perchlorinated aromatic hydrocarbons which consist of six-member rings do not produce molecular ionic peaks, while the other PCAHs give both molecular ionic peaks and adduct ionic peaks.