Cation binding to Na,K-ATPase is characterized in native membranes at room temperature by solid-state NMR spectroscopy using the K+ congener 205T1. It has been demonstrated that the signals from occluded T1+ and nonspecifically bound T1+ can be detected and distinguished by NMR. Effects of dipole-dipole coupling between 1H and 205T1 in the occlusion sites show that the ions are rigidly bound, rather than just occluded. Furthermore, a low chemical shift suggests occlusion site geometries with a relatively small contribution from carboxylate and hydroxyl groups. Nonspecific binding of T1+ is characterized by rapid chemical exchange, in agreement with the observed low binding affinity. textcopyright 2006 American Chemical Society.