The dielectric properties of several ion forms of the synthetic crystalline zeolite Linde type-A have been studied in the frequency range 5 Hz to 100 MHz in the presence of varying amounts of adsorbed water. In addition the mixed Ca/Na form of type-A (i.e. the commercial 5-A) has been studied with ammonia, sulphur dioxide, carbon dioxide and n-pentane as the adsorbed phase in the frequency range 5 Hz to 150 kHz. A cell specially suitable for the above studies has been designed, and constructed, and satisfactorily operated. The cell enables the zeolite to be dehydrated at temperatures up to 5000C under a vacuum of 10-5 torr and tben equilibriated with any required pressure of any gaseous adsorbate before measurement. In all the ion forms studied several regions of dielectric relaxation have been observed. The results for water have been interpreted by considering two relaxation regions as being due to adsorbed water. A small proportion (about 20 per cent) apparently behaves as liquid water, the remainder having an ice-like character. In all the ion forms a region of dielectric loss may be attributed to movement, within the zeolite cavity, of the interstitial cations. The frequency of occurrence of this dielectric loss peak is dependent on the cation, the adsorbate and the adsorbate content. Temperature variation of this peak shows a linear relationship between the activation energy of the process and the radius of the cation concerned. The behaviour of this activation energy with temperature appears to imply a type of phase transition (freezing) of the adsorbed water. Dielectric isotherms for 5-A/I:I20, 5-A/NH3 and 5-A/S0 2 have been obtained and the results discussed in terms of the available smtes for adsorption. The isotherms indicate that a large proportion of 802 appears to have a relaxation time similar to that in the bulk liquid. The different behaviour of H20 and NH3 is attributed to the hydrogen bonding capabilities of the latter. Some variation of the dielectric properties of 4-A between different batches has been observed; this is attributed to varying amounts of sodium aluminate in the material. Values of apparent d.c. conductivity obtained during the course of the dielectric study, while in agreement with previous dielectric studies, are notably different from some published conductivity values. While the investigation shows that studies of dielectric relaxation can give considerable information concerning the adsorbate and adsorbent, results of molecular behaviour by other techniques are required to enable a convincing interpretation of the results to be made; these are not available.