The formation of a complex between (1-->3)-beta-D-glucan and congo red in dilute alkaline solutions (0.1 and 0.15 M sodium hydroxide) where the glucan takes an ordered conformation was studied by measuring visible absorption spectra of the solutions at various temperatures between 15 and 35 degrees C. The average number of the glucose residues forming the binding site to accommodate one dye molecule decreased with increasing temperature or decreasing alkaline concentration, suggesting possible conformational changes of the glucan chain. The complex formation constant, K, increased with increasing temperature, and thermodynamical parameters, delta H degrees and delta S degrees, for the complex formation in 0.1 M and 0.15 M aqueous NaOH solutions were obtained at 25 degrees C to be 14 kcal/mol and 77 e.u. (0.1 M), and 12 kcal/mol and 68 e.u. (0.15 M), respectively. These results indicate that the driving force for the complex formation is entropic in nature stemming from the decrease of "iceberg formation".