A novel change of phase behavior and properties of polyvinyl alcohol (PVA)/gelatin blends as a function of pH was reported. The PVA/gelatin blends were found to be completely miscible in acidic condition (pH < 4), partially miscible in basic condition (pH > 8), and immiscible in neutral condition (pH was ca. 6). As a result, the membranes cast from acidic condition showed the highest tensile strength and the lowest alcohol vapor permeation (AVP) rate; those obtained from neutral condition showed the lowest tensile strength and highest AVP rate; the properties of membranes cast from basic condition lay in between. The interaction between PVA and gelatin was investigated via Fourier transform infrared spectrum (FTIR), differential. scanning calorimetry (DSC), and Zetasizer measurement. The novel pH-dependence of the blends was ascribed to the protonation of amino groups of gelatin in acidic condition, which resulted in a strong electrostatic attraction between -NH2+ of gelatin and -OH of PVA. The partial miscibility in basic condition was due to the ionization of carboxyl groups of gelatin, which caused a stretching of gelatin via electrostatic repulsive force and a breakage of the H-bonding among the molecular chains, leading to a limited interaction between PVA and gelatin and forming a partially miscible blend. In neutral conditions, there were almost no charges (very limited protonation and ionization) at the weak polyampholyte gelatin, and the strong H-bonding among gelatin molecules themselves or PVA molecules themselves caused the phase separation between gelatin and. PVA. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 239-247, 2009