In mixed sodium-rubidium berate glasses with a constant total content of 30 mol% alkali oxide and various relative concentrations of sodium and rubidium diffusion of Na-22 and Rb-86 was studied at 673 K using the radiotracer serial sectioning method. The diffusion coefficients of sodium and rubidium show a concentration dependence which is at variance with 'classical' expectations for mixed-alkali glasses. No cross-over of the diffusivities is observed since Na diffusion is always faster than Rb diffusion. Na diffusion passes through a minimum near 60% Na2O content whereas Rb diffusion shows a maximum near 50% Na2O content. The ionic conductivity deduced thereof displays a minimum as a function of the relative composition of sodium and rubidium like in 'classical' mixed-alkali glasses. (C) 2000 Elsevier Science B.V. All rights reserved.