Humic Substances (HS) are ubiquitous in natural waters, Due to their plentiful binding sites, HS play an important role in the mobilization and transport of pollutants in natural environments. However, the presence state of HS causes difficulty in exploring the interactions between HS and pollutants in aqueous solutions. In this Study, an effective and rapid method was proposed to quantify the interaction between a cationic dye and HS in aqueous solutions. Humic acids (HA) and Toluidine Blue (TB) were respectively used as the model HS and model cationic dye. This characterization method was based on the binding of TB to HA molecule to form a TB-HA complex. From the difference between the visible spectra of the dye and the TB-HA complex, the adsorption capability of HA could be evaluated. Results demonstrate that the Langmuir adsorption isotherm theory was appropriate to describe the interaction between TB and HA. Both pH and ionic strength had significant effects on the adsorption of TB onto HA. These results suggest that the binding reaction was a physical adsorption driven by the electrostatic interaction. (C) 2008 Elsevier Inc. All rights reserved.