A nepheline hydrate I crystal, ion-exchanged using KCl(aq) at 80°C, was found to be orthorhombic, space group Pnm2 1, a = 8.113(3), b = 15.223(2), c = 5.1817(7)Å and showed no superlattice X-ray reflections. Structure analysis by means of Fourier and least-squares methods led to the composition K 1.1Na 1.9Al 3Si 3O 12 · H 2O ( Z = 2, Dc = 2.40 g cm -3) and the agreement factors: R = 0.032 and Rw = 0.040. Species assigned to the observed extra framework sites were K(1), K(2), and W(1) in the 8-ring tunnels along c plus Na(1) and Na(2) in the smaller 6-ring voids forming connections in the b direction. The atoms Na(1) and Na(2) coordinated framework oxygens exclusively and were but little affected by the ion-exchange process; K(1) was found near the center of an 8-ring and had five O atoms and two water molecules as closest neighbors, while the weakly occupied K(2) site was near a 6-ring and was found to have a coordination consisting of at least five oxygens and one water. The 10% vacancy of Na(2) is compensated for by an equal amount of K(2), which does not enter the Na(2) site for sterical reasons.