Enthalpies of solution have been measured in 20.1 weight % HF at 50 degrees C on synthetic nepheline and kalsilite samples having 0.125 excess silicon atoms per four oxygen atoms via (Na,K)Al = Si substitution (where  = vacancies), then compared with data for two series of nepheline-kalsilite samples having less excess silicon. Differences in the enthalpies of solution between the new samples and those having no excess silicon are 8.4 and 14.4 kJ/mol, respectively, for potassium-free nepheline and sodium-free kalsilite, energy rates that are comparable to those of K-Na substitution. Enthalpies of K-Na mixing, as well as the critical temperature of the solvus, also are increased by the higher Si content. Considering both the tendency of these solid solutions to contain excess silicon and the energetic consequences of that substitution, we suggest that they routinely be treated as ternary. This study documents the significant thermodynamic effects of relatively minor chemical substitution on both end-member data and mixing properties, and speaks to the need for through chemical characterization of phases on which thermodynamic and phase equilibrium studies are conducted. The application of end-member thermodynamic data to natural phases may be inappropriate, even if chemical differences between the two are small.