A complete crystallographic study and the ionic distribution in the conduction layers, including H2O, of polycrystalline Cd2+-stabilised K+-beta/beta " -ferrite, and Mg2+, Ca2+ ionic exchange derivatives is presented. Both the parent compound and the ionic exchange products are not single phases, but present a mixture of the beta "- and beta -phases. The weight percent of water molecules calculated from Rietveld analysis is 0.96% for K+, and 2.41% for Mg2+ and Ca2+, in good agreement with the weight losses measured by thermogravimetric analysis of 1.5, 2.2 and 2.5%, respectively.