The step-wise ion exchange replacement of sodium by either potassium or strontium has been studied for the two highly aluminous zeolites MAX (maximum aluminium X or LSX, low-silica X) and MAP (maximum aluminium P) to determine ion exchange selectivities. Powder diffraction data were gathered at different degrees of exchange and used to refine the structures of hydrated MAX exchanged with strontium (powder X-ray diffraction) or potassium (powder neutron diffraction). The cubic lattice of MAX expanded (from 25.06 angstrom to 25.15 angstrom) when 80% metal loading (K+ or Sr2+) was achieved. Concurrent measurements of ion exchange selectivity showed a sharp decrease in selectivity coefficients at 80% metal loading. These results and evaluation of earlier work indicates that structural changes (e.g. lattice expansion) may be a major cause for the strong non-ideality of ion exchange selectivity, commonly observed in high aluminium zeolites ion exchange. Changes in solution pH may also be an important factor affecting the ion exchange selectivities because the dissociation of OH-groups the weakly acidic zeolites is sensitive to solution pH. (c) 2006 Elsevier B.V. All rights reserved.