AbstractThe thermal and hydrothermal stability of the zeolite NaCaA in dependence on the ion exchange degree have been investigated. The increasing thermal stability of the zeolite with increasing content of calcium ions corresponds to the higher stability of calcium silicates compared with sodium silicates. In contrast to that the decreasing hydrothermal stability is due to the decreasing alkalinity of the zeolite in aqueous media. Measurements of the pH‐value of zeolite/water slurries underline this tendency.