Two samples of Na-substituted leucite with Na/(Na + K) = 0.74 (1) and 0.89 (2) were studied by dilatometry, thermogravimetry, and X-ray powder diffraction method. Sample 1 is a mixture of analcime and leucite, and sample 2 consists of analcime alone. The miscibility of K+ and Na+ in hydrated analcime is estimated at 15–20 at.% K+. The thermal behavior of the samples varies depending on their Na/K ratio. Dehydrated Na,K-analcime undergoes a trigonal distortion; its structural model is proposed. The earlier recognized difference in the thermal behavior of primary and secondary (obtained from leucite through K+–Na+ exchange) analcimes might be due to the influence of unsubstituted K+.