The influence of the water state and grafted polystyrene content (from 8 to 71 wt%) on ionic conductivity of cation-exchange membranes based on polyethylene and sulfonated grafted polystyrene has been investigated. Such membranes possessed a relatively high proton conductivity both in humid (up to 6.6-10(-3) Omega(-1)cm(-1) at RH = 95%) and dry states (up to 1.8.10(-3) Omega(-1)cm(-1) at RH = 32%). Membrane saturation with water led to a dramatic increase in the ionic conductivity. FT-IR spectroscopy, DSC, and membrane proton conductivity measurements in the temperature range from -30 to 80 degrees C were used to investigate the evolution associated with the state of the water molecules and proton hydrates mobility. It turned out that at different degrees of hydration, the mobility of proton-containing groups changed in a different manner. At low humidity, a smooth mobility increase was observed. For membranes in highly hydrated state, a phase transition occurred at a temperature of about 0 degrees C, associated with the "melting" of a significant part of water molecules located in the center of the pores. However, this transition did not lead to an abrupt increase of the membrane ionic conductivity because such water molecules did not contain protons. (C) 2017 Elsevier B.V. All rights reserved.