Abstract Aspects of osmotic properties of liposomes, prepared from synthetic lecithin, above, at and below the gel to liquid crystalline phase transition temperature are described. The experiments show that liposomal membranes with their lipids in the gel state are still permeable to water. The rate of water permeatation changes drastically on passing the transition temperature. The water permeatation has activation energies of 9.5±1.28 and 26.4±0.9 kcal/mol above and below the transition temperature, respectively, indicating that the diffusion process take place by different mechanisms. With respect to the barrier properties of the liposomes in the vicinity of the transition temperature, the following conclusions can be made. 1. (1) Studying the osmotic shrinkage of liposomes at a fixed temperature near the transition point, the experiments indicate that dimyristol phosphatidylcholine membranes are higly permeable to glucose under these conditions, where liquid and solid domains co-exist. Under the same conditions the osmotic experiments did not indicate a strong increase in glucose permeability of dipalmitoyl phosphatidylcholine membranes as compared to the situation above and below the transition temperature. 2. (2) On the other hand, perturbations of the phase equilibrium by temperature variations resulted in a marked increase of the glucose permeatation through dipalmitoyl phosphatidylcholine bilayers. Once a new phase equilibrium of liquid and solid regions is established the permeatation rate of glucose is much leass.