A series of single-phase alkali feldspars in the system Na2O-K2O-Al2O3-SiO2-H2O were crystallized from dehydrated gels at 500 bars and 800° C, in the presence of an aqueous vapor phase, quartz, and corundum. Twenty of these bulk samples were analyzed for K2O and Na2O by atomic absorption spectrophotometry. The unit cell parameters for 19 of these samples and 8 others in the series were obtained by least squares refinement of x-ray powder diffraction data collected with a focusing camera. Polynomial expressions giving the variation in unit cell parameters as a function of composition, in mol fraction potassium feldspar, were obtained, and compared with similar polynomial expressions developed from the avilable data on the sanidine-high albite series.