AbstractCardamonin (2′,4′‐dihydroxy‐6′‐methoxychalcone), one of the main constituents from the seeds of Alpinia katsumadai Hayata, belongs to chalcone with its antibacterial, antiinflammatory and other important therapeutic activities of significant potency and low systemic toxicity. In this article, the interaction of cardamonin to human serum albumin (HSA) has been studied for the first time by spectroscopic methods including Fourier transform infrared (FTIR) spectroscopy, circular dichroism (CD), and UV‐absorption spectroscopy in combination with fluorescence quenching under physiological conditions with drug concentrations of 0.67–4.0 μM. The results of the spectroscopic measurements and the thermodynamic parameters obtained (the enthalpy change ΔH0 and the entropy change ΔS0 were calculated to be –25.312 and 7.040 J·mol–1·K–1 according to the van't Hoff equation) suggest that hydrophobic interaction is the predominant intermolecular forces stabilizing the complex, which is also in good agreement with the results of the molecule modeling study. The alterations of protein secondary structure in the presence of cardamonin in aqueous solution were quantitatively calculated by the evidence from CD and FTIR spectroscopes with reductions of α‐helices of about 20%, decreases of β‐sheet structures of about 14%, and increases of β‐turn structures of about 15%. The quenching mechanism and the number of binding sites (n ≈ 1) were obtained by fluorescence titration data. Fluorescent displacement measurements confirmed that cardamonin binds HSA on site II. In addition, the effects of common ions on the constants of the cardamonin–HSA complex were also discussed. 2005 Wiley Periodicals, Inc. Biopolymers 79: 48–57, 2005This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com