Molecular complexes formed between a cooked food mutagen-carcinogen (2-amino-3-methylimidazo[4,5-f] quinoline, IQ) and a potent anti-mutagen (4-hydroxy-coumarin, 4HC) were synthesized and crystallized from a solution. The active functional groups in IQ and in 4HC that may involve in the formation of these complexes are investigated by UV-visible, infrared, and NMR spectroscopy. The geometrical conformations and heat of formation of IQ:4HC complexes were optimized by semi-empirical molecular orbital calculations. The experimental and theoretical results indicate that the complexes exist in multiple conformations. The energetically favorable ones are formed via hydrogen-bonding type of interactions between amino group in IQ and carbonyl group in 4HC, and also between quinoline-nitrogen (and imidazole-nitrogen) in IQ and hydroxyl group in 4HC. The observed complex formation could serve as a model for understanding the active molecular sites responsible for the initial mutagenic activity in cooked-food mutagens as well as the anti-mutagenic activity in coumarins.