Complexes from poly(vinylbenzyltrimethylammonium chloride) and polystyrene-sulfonate are transparent, water-insoluble materials which offer interesting possibilities in ophthalmology for contact lenses and surgical implants. The permeability to water of a series of neutral polyelectrolyte complex membranes was determined. The measurements were carried out at relatively low pressure gradients (20–900 mm. Hg). The results are in good agreement with previously reported data obtained at higher pressures (6.9 atm. and higher). The refractive index of the complexes was also determined, and apparently a linear relationship exists between the refractive index and the amount of water in the complex: 70% water, nD 1.40; 45% water, nD 1.48; 35% water, nD 1.51. A polyelectrolyte complex membrane 0.09 mm. thick containing 60% water at equilibrium swelling will have a similar permeability to water as the endothelium of the cornea (1.55 × 10−11 cm.3/sec.-dyne) at 35°C. The same membrane, but with a thickness of 0.03 mm., will have a permeability similar to that of the whole corneal stroma (4.9 × 10−11 cm.3/sec.-dyne). At 0.21 mm. thickness the membrane will have a permeability similar to that of the epithelium (0.67 × 10−11 cm.3/sec.-dyne).