Eleven nepheline–kalsilite crystalline solutions with various proportions of K:Na have been studied from room temperature to 1050/1150°C by X-ray powder diffraction. Nepheline expansion is relatively high and little affected by composition, whereas kalsilite expansion is lower but affected to a significant degree by K:Na ratio. The generally higher rate of expansion in nepheline is apparently related to the collapse of the tetrahedral framework around the smaller of its two alkali sites. Occupancy of these sites by the relatively small Na ion further extends the potential for thermal vibration before the structure is stretched to the critical degree required for phase transformation. Once the structure changes to that of kalsilite, with its single alkali site, an increase in content of the larger K ion limits the degree to which kalsilite can expand. Crucial to the overall expansion behaviour of these minerals are the specific tetrahedral configurations of nepheline vs. kalsilite, the number and geometry of their alkali sites, the occupancies of those sites, and the flexibility inherent in each structure that allows for adjustment with increasing temperature.