Creatinine was found to not only act as a precursor of 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP) formation but also inhibit PhIP formation in a creatinine/phenylalanine model system. The dual mechanistic effects of creatinine on PhIP formation were then investigated in a model system. Adducts of creatinine-PhIP were detected by quadrupole-time-of-flight mass spectrometry and were found to be a likely explanation for the substantial decrease in the yield of PhIP when excess creatinine was supplied. Structures of probable adducts were predicted in molecular docking studies, which showed that hydrogen bonds were formed between creatinine and PhIP in 1:1 and 2:1 ratios. Furthermore, the active sites during creatinine-PhIP adduct formation (the primary amino groups [N2 -] and sp2 nitrogen atoms [N3 ] of creatinine and PhIP) match the active sites of PhIP metabolism and adducts of PhIP/lipid-derived reactive carbonyls. This verifies that creatinine inhibits PhIP production via the formation of adducts with hydrogen bonds at the N2 and N3 sites.