This paper describes the preparation and testing of various ionic forms of clinoptilolite, a natural zeolite, and a study of their properties for the removal of water vapour from air. The calcium form of the zeolite showed the highest uptake of 0.127 g of water per gram of zeolite and the potassium form was the worst with a capacity of half this value. The sorption data were correlated by a 'Langmuir' isotherm. The size of the cation sorbed had a direct effect on the water sorption data. The results are compared with the properties of other desiccants. The zeolite uptake of water is less than that of silica gel. (C) 1997 Elsevier Science B.V.