A process for the preparation of a coated substrate is described, in which a substrate is coated with a coating mixture containing a polymer and a layered double hydroxide, the layered double hydroxide having been derived from a layered double oxide. The process of the invention is markedly simpler than conventional techniques for affording coated substrates having reduced permeability to degradative gases. The coated substrates obtainable by the process are particularly useful in packaging applications, notably in the food industry.