Isopiestic and viscosity measurements on aqueous sodium cholate-lecithin solutions are reported at high concentrations which include the physiological region as a lower limit. At low cholate concentration, the addition of lecithin increases the activity of water, consistent with earlier studies showing increases of micellar weights. At higher cholate concentrations, the activity of water is independent of lecithin concentrations. Studies of binary solutions of bile acid salts and detergents agree very closely with earlier work.