The characteristics of host – /guest complexation between hydroxypropyl-β-cyclodextrin (HP-βCD) and three forms of coumarin,i.e.coumarin,7-hydroxycoumarin (7-HC) and 6-methylcoumarin (6-MC),in water and phosphate buffer solution of pH 7.4 at different temperatures (from 298.15 K to 313.15 K) were investigated by phase solubility method.The phase-solubility diagrams drawn from UV spectral measurements were of the AL-type as shown in figure 1 [1,2].The solubilities of coumarin and its derivatives in aqueous HP-βCD solutions increased with the ascent of the temperature at the same concentration of HP-β-CD.The apparent stability constants (Ka ) of inclusion complexes were calculated based on the solubility data and followed the order of 7-HC > 6-MC > coumarin in the same solvent at the same temperature.And the stability constant of 7-HC in water was larger than that in the phosphate buffer solution,which was contrary to the change tendency of 6-MC and coumarin.The relevant thermodynamic parameters including the standard Gibbs free energy change (ΔG),the standard enthalpy change (ΔH),and the standard entropy change (ΔS) were determined from the van’t Hoff plot.The enthalpy change and entropy change of the inclusion process are negative,which showed that the inclusion interactions were enthalpy – driven process.All the results were interpreted by the driving forces occurred in the inclusion complexation.