The surface and structural properties of synthetic zeolites thermally treated between 100 adn 1000°C were investigated by X-ray, DTA, TG and nitrogen adsorption at −195°C. The zeolites under investigation possess a crystalline structure similar to nepheline; their diffraction patterns completely disappeared on thermal treatment at 800°C due to destruction of the zeolitic lattice. Differential thermal analysis exhibited an endothermic effect centered at about 225°C which may be attributed to dehydration of loosely bound zeolitic water, and two successive exothermic effects; the former was small, centered at 855°C, the latter, which was sharper, appeared at about 920°C. These may be due to evolution of structural water followed by destruction of the zeolite lattice, respectively. Thermogravimetric analysis indicated that the water loss in air exceeded that in vacuum, an effect which is indicative of contraction of the zeolite structure by the action of a high vacuum procedure at elevated temperature. Nitrogen adsorption studies at −195°C were used as a probe to follow the textural and structural changes due to thermal treatment between 100 and 1000°C. Evolution of the monolayer equivalent using the method of Dubinin Radushkevich gave higher estimated values than those calculated by the BET method. These higher values can be attributed to the influence of the micropore filling of nitrogen in the low pressure region which led to distortion in the shape of the isotherms in the initial pressure region, and hence the evaluation of V m by the BET method may be considered unreliable in this connection. In general, monolayer equivalents increase with increase in the pretreatment temperature up to 400°C followed by a gradual decrease as the pretreatment temperature increases. The increase in monolayer equivalents may be correlated with the removal of loosely bound water in the intracrystalline cavities or pores, an effect which leads to an increase in the accessibility of the nitrogen molecule. The decrease in nitrogen uptake on samples thermally treated at temperatures higher than 400°C can be attributed to contraction of the zeolite pores by heating in vacuum: an effect which precedes partial collapse of the structure. The marked decreased obtained on thermal treatment at 800°C may be correlated with destruction of the zeolite lattice producing a mixture of different sintered oxides.