In order to make clear the adsorption mechanism as well as the effective hydrophobicity of four kinds of sodium salts of bile acids (cholate (NaC), deoxycholate (NaDC), chenodeoxycholate (NaCDC) and ursodeoxycholate (NaUDC)), the effect of temperature on the adsorption on graphite (12.9 m 2 g −1) was examined for these bile salts in tetraborate-carbonate buffer solution (at pH 10.0). The isotherms observed exhibit a two-step adsorption below 30°C, but above 37°C monolayer adsorption takes place, suggesting that there exists a certain transition temperature. In some cases a further increase in adsorption, of a BET-type, was observed at higher concentrations. From the Langmuir plot, the maximum adsorption amounts of monolayer, N m, the constants of adsorption equilibrium, b, and the mean surface areas, A m, occupied by a bile salt molecules were determined as a function of temperature (at 25, 30, 37 and 42°C). The van't Hoff plot was applied to estimate the heat of adsorption, − ΔH, of each bile salt on graphite. The decreasing orders of − ΔH and N m, were found to be NaCDC>NaC>NaDC>NaUDC, and NaDC>NaCDC>NaC>NaUDC, respectively. The disagreement between the orders is discussed.