Water-based formulations are preferred for membrane coat application because they do not require the use of noxious solvents. A novel aqueous chitosan± alginate±CaCl 2 system was evaluated as a potential formulation to produce waterinsoluble membranes of biodegradable polymers. Chitosan±alginate coacervates were prepared by controlled reaction of chitosan (0.25% w/v) and sodium alginate (0.25% w/v) solutions. Coherent membranes were obtained by casting and drying the coacervates suspended in aqueous CaCl 2 solutions (0.05±0.07% w/v). Increasing the calcium content did not modify membrane thickness (25±26 mm), but reduced the water vapor transmission rate from 658 to 566 g/m 2 /day, and improved the tensile strength of the membranes from 9.33 to 17.13 MPa. Differential scanning calorimetry, Fourier transform infrared spectroscopy, and elemental analyses of the chitosan±alginate coacervates indicated they were stable for up to 4 weeks of storage in distilled water at ambient temperature. Membranes of the stored coacervates required less calcium to attain maximum mechanical strength. They also had higher water vapor transmission rates than corresponding ®lms prepared from fresh coacervates. On the basis of the properties of the cast ®lm and its storage stability, the chitosan±alginate±CaCl 2 system can be considered for potential membrane coat application.