Ion-exchange membranes allow ions to pass through them, and are needed for applications such as fuel cells. Some water must be present in the membrane to support ion transport, but water is often lost from membranes in conditions of low humidity and/or high temperature. Park et al.1 have prepared membranes coated with a thin layer of a hydrophobic material that contains cracks. At high humidity, the membrane swells as it absorbs water (white arrows indicate water movement), mainly through the cracks, allowing ion transport. At low humidity, the membrane loses some water and contracts. This substantially closes the cracks and slows water loss, helping the membrane to retain moisture and maintaining efficient ion transport even in dry conditions.