Method to improve the barrier properties of composite gas cylinders and high pressure gas cylinder having enhanced barrier propertiesThe instant invention pertains to a new method for improving the barrier properties of composite gas cylinders for the storage of gas, by sheathing the outer surface of a composite gas cylinder with a tubular plastic film comprising a barrier material and shrinking it by heat treatment to approach a strong connection. The composite gas cylinder comprises an inner liner made of polyolefin and an outer fibre-reinforced, pressure supporting layer. The barrier material may comprise polyamide, polyester, halogen substituted polymer, EVOH or a metallization. The invention pertains also to a high pressure composite gas cylinder having enhanced barrier properties and its use as a fuel tank in gas driven automotive vehicles equipped with a combustion engine.