To apply the adsorption/desorption behavior of water on zeolite to a heat storage system which can generate both warm and cold heat, equilibrium data under low relative vapor pressure were measured. Based on the data the adsorptivities under adsorption/desorption are estimated. Heat storing/releasing characteristics were also studied by using an experimental heat storage device with 20 wt-% sodium chloride solution and zeolite 13 X. Heat storing/releasing cycles which have adsorptivity higher than 0.1 kg . kg(-1) per cycle even under relative vapor pressure as low as 0.1 and an evaporation process at temperatures lower than 0 degrees C, are achieved. A water-zeolite type heat storage system using a slight amount of sodium chloride for lowering the solidification point of water is proposed for heating and freezing applications.