The solubility of daidzein in propylene glycol + water cosolvent mixtures was determined by UV spectrophotometry, and predicted using the Jouyban-Acree model and the mathematical model based on the algebraic rule of mixing. The thermodynamic functions, such as Gibbs energy, enthalpy, and entropy of solution and of mixing and so on, were obtained from these solubility data by using the van't Hoff and Gibbs equations. The results show that the daidzein solubility increases as propylene glycol proportion increases in the mixtures and the solution temperature increases. The Jouyban-Acree model can be used to predict the solubility of daidzein in propylene glycol + water cosolvent mixtures at different temperatures. The driving mechanism for daidzein solubility in water-rich mixtures is the entropy, probably due to water-structure loss around the drug non-polar moieties by effect of propylene glycol, whereas the driving mechanism is the enthalpy above 0.20 mass fraction of propylene glycol, probably due to daidzein solvation increase by the cosolvent molecules. Non linear enthalpy-entropy compensation with negative slope from water up to 0.20 mass fraction of propylene glycol and positive slope beyond this composition up to pure propylene glycol is found. (C) 2014 Published by Elsevier B.V..