The rate of swelling of egg lecithin liposomes under osmotic shock has been studied employing a stopped-flow spectrophotometer. Incorporation of cholesterol and simple alcohols into the liposomal structure elicits a biphasic response in swelling rate: at low concentrations these additives increase but at high concentrations they decrease water permeabilty. For simplen-alkanols, the effects can be correlated with structure. Specifically, the concentration of alcohol required to elicit maximal permeability as well as the maximal permeability decreases with increasing length of the alcohol. These effects are accounted for on the basis of modification of the orientation and packing of lecithin molecules in the bilayer membrane of the liposome.