Article gives analysis of experimental results of water absorption by expanded polystyrene slabs. The investigation was carried out applying different test methods. It was established that the most precise results of water absorption by expanded polystyrene are obtained when method of increased pressure is applied. Results obtained applying this method show that the water absorption of expanded polystyrene considerably increases when its density decreases. When the specimen is immersed, the largest amount of water is accumulated between the pores. Due to an even pressure distribution around the walls of the specimen, water does not push out the air, which is between the pores, and, therefore, a part of the specimen does not soak through. Moreover, spaces between expanded polystyrene pores usually interact, and, depending on the density of a product, the diameter of them may be up to few millimetres. When the specimen is withdrawn from water, all of a sudden the greatest amount of water retires from the spaces between the pores
