This International Standard specifies a method for the determination of the water absorption of rigid cellular plastics by measuring the buoyant force on a test specimen after immersion under a 50 mm head of water for 4 days. Corrections are specified to take account of any change in volume of the specimen and also to correct for the volume of water in the cut surface cells of the specimen. The water absorption is expressed as the average, for several specimens, of the percentage increase in volume relative to the original volume. The method described is intended for quality control and for use in product specifications.