Inclusion complexation of various barbituric and thiobarbituric acid derivatives with β-cyclodextrin in aqueous solution was studied by ultraviolet (UV), circular dichroism (CD), and nuclear magnetic resonance (NMR) spectroscopies. It was found that the induced CD observed for barbituric acids were substantially different from those for thiobarbituric acids. Chemical Shifts changes in 1H-and 13C-NMR spectra suggested that not only 5-substituent but also heterocyclic moiety of barbiturate participated in inclusion formation. The stability constant of the inclusion complex was determined by UV method at various pH and temperatures from which the thermodynamic parameters were determined. Free barbituric acids were more favorable to form inclusion complex than ionized form. The relationship between the number of carbon atoms in 5-substituents and the formation constant were sigmoidal but barbituric acids bearing cyclic substituent at 5-C deviate from the correlation curves. Isoequilibrium relationship between ΔH and ΔS values were observed having a compensation temperature of 372°K.