The binding interaction between two dicationic styrylimidazo[1,2-a]pyridinium dyes and human serum albumin (HSA) was investigated at physiological conditions using fluorescence, UV-vis absorption, and circular dichroism (CD) spectroscopies. Analysis of the fluorescence titration data at different temperatures suggested that the fluorescence quenching mechanism of HSA by these dyes was static. The calculated thermodynamic parameters (G degrees, H degrees and S degrees) indicated that hydrogen bonding and van der Waals forces played a major role in the formation of the dye-HSA complex. Binding distances (r) between dyes and HSA were calculated according to Forster's non-radiative energy transfer theory. Studies of conformational changes of HSA using CD measurements indicate that the -helical content of the protein decreased upon binding of the dyes. Copyright (c) 2016 John Wiley & Sons, Ltd.