Thin film electrodes for lithium ion batteries (LIB) poses several attractive advantages over traditional composite electrodes including size and shape constraints, operating temperature range, and volumetric energy density. Tin is an attractive candidate for LIB anode applications due to its exceptional specific capacity, cascading voltage profile, safety, wide availability, and low cost. Tin thin film electrodes were sputtered onto the current collector of a recently developed in situ X-ray diffraction (XRD) and were monitored continuously by XRD while cycling. A phase transformation from white tin, to Li2Sn5, to beta-LiSn, to Li22Sn5 was observed during lithiation with the same phases detected in reverse order during delithiation. The Li2Sn5 phase is not seen in the high temperature phase diagram of the Li-Sn system. Preferred orientation and crystallite size information for these phases was extracted from the XRD data in order to develop a clearer picture of how lithium enters and exits thin film tin electrodes. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.077203jes] All rights reserved.