The electrochemistry and the structural changes that occur during sodium insertion and removal from tin are studied by in-situ X-ray diffraction at 30 degrees C. The Sn vs. Na voltage curve has four distinct plateaus, corresponding to four two-phase regions during sodiation, and indicating that four Na-Sn binary alloys are formed. The alloy formed at full sodiation was found to be Na15Si4, as expected from the Na-Sn binary system at equilibrium. The three intermediate Na-Sn phases that form during sodiation have X-ray diffraction patterns that do not correspond to any known equilibrium phase of Na-Sn. More work is needed to characterize these new binary Na-Sn phases. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.037211jes] All rights reserved.