Chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs) as disinfection products for source waters containing PAHs have raised environmental concern due to their dioxin-like toxicity and potential hazardous effect on human health. In this study, we investigated the reactivity and transformation product distribution of eight environmentally relevant PAHs, namely, naphthalene (Nap), acenaphthylene (Acy), acenaphthene (Ace), fluorene (Fl), phenanthrene (Phe), anthracene (Ant), fluoranthene (Flura) and pyrene (Py), during chlorination. Our results showed that Acy, Ant, Ace, and Py exhibited higher reactivity than Nap, Fl, Phe, and Flura, and their reactivities were reasonably related to their chemical hardness. Transformation products such as chlorinated and oxygenated PAHs were first predicted by electronic structure theory, then qualitatively identified by comprehensive two dimensional gas chromatography-quadrupole mass spectrometry technique, and finally quantitatively confirmed by the "tailor-made" reference compounds we synthesized. Ace and Py were found to be the major precursors of Cl-PAHs, and their chlorination mechanisms were elucidated based on the observational evidence for their mass balance. However, Acy and Ant mainly produced less toxic oxygenated PAHs. PAHs that tend to generate oxygenated transformation products seem to manifest higher reactivity than those form Cl-PAHs due to different reaction mechanisms: single electron transfer followed by a nucleophilic addition for oxygenation vs. electrophilic attack for chlorination. This study provides a fundamental mechanistic basis for better understanding of chlorination product formation process, and guides application of chlorination technology.