The behaviour of poly(vinyl methyl ether) (PVME) in water has been studied in detail. A molecular complex with maximum two molecules of water per repeating unit is formed. The glass transition temperature of this complex is -58°C. Additional water plastisises and hydrates the complex. This conclusion is based on the detailed analysis of the concentration dependence of the glass transition of the system and melting behaviour of water. A similar behaviour is found with the chemical networks, obtained by radiation cross-linking.