Specimens of both natural (Monte Somma) and synthetic Na-rich nepheline have been used to synthesize two ion-exchange series, ranging in composition to pure kalsilite. Excess Si in the natural and synthetic series was 5.2 and 1.7 mol%, respectively. The associated vacancies (VAC) in the alkali sites were found to play an important role in the crystal chemical behavior of these minerals. Room temperature X-ray powder diffraction measurements revealed the presence of three phases in each series: nepheline from sodic compositions to 0.64 (± 0.01) mole fraction K + VAC, tetrakalsilite at 0.72 (± 0.02) K + VAC, and kalsilite above 0.80 K + VAC, with a narrow miscibility gap between K-rich nepheline and tetrakalsilite. Increases of a, c, and volume were linear with K content within each structural region. However, for nepheline the increases accelerated beyond compositions at which K (or K + VAC) was forced to occupy Na structural positions. For the more vacancy-rich natural series this occurred at lower K contents, suggesting preference of vacancies for the larger alkali position. The overall effect of ☐Si exchange in nepheline, in fact, was found to resemble that of large-ion substitution in the alkali site. Both the nepheline to tetrakalsilite and tetrakalsilite to kalsilite transitions were accompanied by apparent discontinuities in the c unit-cell dimension. Unit-cell volume was discontinuous for a hypothetical nepheline to kalsilite transition. Magic-angle-spinning (MAS) NMR data of 23Na, 27Al, and 29Si for part of the natural series suggest that the nepheline structure is distorted on both a local (unit-cell) and long-range scale with the addition of K. Similarly, the kalsilite structure is distorted with increasing Na content. Stoichiometric nepheline (K + VAC = 0.25) is observed to have the least distorted tetrahedral sites as calculated from the 27Al quadrupolar coupling constant. Lastly, the 29Si and 27Al NMR suggest complete ordering between the Si and Al tetrahedral sites, thus preserving the Al avoidance rule.