The competition of Na+ ion versus K+ ion transport in a mixed alkali borosilicate glass has been investigated by low energy bombardment induced ion transport employing Cs+ ions as the foreign ion. Electrodiffusion causes the replacement of Na+ and K+ down to about 200nm below the surface of the glass. Beyond this electrodiffusion front (in the direction of ion transport) K+ ions accumulate to a density above the bulk concentration while Na+ is further depleted towards the backside electrode. At the backside electrode only Na is electrodeposited since the electrical potential does not allow for K electrodeposition. A full simulation of the electrodiffusion profiles reveals the complete concentration dependence of the diffusion coefficients of the Na+ and K+ ions.