Reflection spectra of mixed-cation (Na1-xKx, Na1-xAgx and Na1-xTlx) beta-aluminas were measured in the ultraviolet and vacuum-ultraviolet regions using a beam line with synchrotron radiation in order to study the mixed-cation effect in beta-aluminas. A new exciton band in the reflection spectra of the mixed-cation Na1-xKx and Na1-xAgx beta-aluminas was observed at the concentration region where the ionic conductivity shows a minimum, in addition to the exciton band of the end components (x=0, 1). No additional band was observed in Na1-xTlx beta-aluminas. The new exciton band correlates to a special structure of the ionic conduction plane in the samples. It is believed this special structure blocks ionic conduction and it is concluded that it is related to the origin of the mixed-cation effect in beta-aluminas. (C) 2004 Elsevier B.V. All rights reserved.