The sodium potassium pump (Na+,K+-ATPase) shows a high selectivity for K+ over Na+ binding when in the E2P conformation, in which the ion transport sites have access to the extracellular medium. To understand the K+ extracellular selectivity in the presence of a high concentration of competing Na+ ions requires consideration of more than just the ion binding affinities. We compared equilibrium-based calculations of the extracellular occupation of the Na+,K+-ATPase transport sites by Na+ and K+ to fluxes through Na+ and K+ transport pathways based on a recently published kinetic model1. The results show that, under physiological conditions, there is 99.7% selectivity for pumping of K+ from the extracellular medium into the cytoplasm relative to Na+, whereas equilibrium calculations alone predict only 20.0% selectivity for K+. Thus, kinetic effects are dominant in determining K+selectivity.