X-Ray structure analysis of a nepheline hydrate I crystal, Rb +-exchanged at 80°C, was performed making use only of the main diffractions. The resulting substructure was found to be orthorhombic with a = 8.0802(8), b = 15.259(2), c = 5.1584(5) Å, V = 636.0Å 3, space group Pnm2 1. Fourier and least-squares techniques gave the residuals R = 0.048 and Rw = 0.058, and a tentative formula of RbNa 2Al 3Si 3O 12 · H 2O ( Z = 2, Dc = 2.65 g cm -3). Tetrahedral distances were consistent with Al,Si alternation in the framework. Of the channel species, Na(1) and Na(2) were found not to be exchangeable at the current temperature. These sodium atoms are located in the small cages, formed by 6-rings of O atoms which connect the 8-ring channels parallel to c into two-dimensional pore systems. In the larger tunnels the replacement was complete and these contain a Rb + ion and probably a water molecule in symmetry-related positions. According to this model, Rb + coordinates four oxygens of an 8-ring and two water molecules, with RbO distances in the range 2.81-3.36 Å. Additional O atoms are found at greater distances.