Na43+ ionic clusters, probably located inside sodalite cages, are formed when zeolite Na+-Y is exposed to Na or K vapour and K43+ cluster when K+-Y is exposed to the vapour of Na or K, but Rb43+ cannot be observed (by e.s.r. spectroscopy) using Rb+-Y under comparable conditions; at higher concentrations of metal vapour intracrystalline metallic particles are formed.