During the electrochemical reaction of lithium with tin, there is sometimes anomalous high-voltage irreversible capacity characterized by an irreversible plateau at about 1.6 V vs. lithium. This has been attributed to electrolyte decomposition catalyzed by pure tin surfaces. Here, we show evidence for this catalytic reaction by in situ atomic force (AFM) and optical microscopy. During the early stages of the anomalous high-voltage plateau we have observed the formation of a film of decomposition products by both in situ AFM and optical microscopy. If lithium is supplied to the tin electrode at a rate greater than the catalytic process can support, then the cell potential decreases and Li-Sn alloys can form. Therefore, by subjecting the Li/Sn cell to a rapid initial discharge to 0.8 V the formation of this film can be suppressed, as we prove with AFM experiments. This rapid discharge to 0.8 V forms the first Li-Sn phase (Li2Sn5) which apparently has no catalytic surfaces that decompose electrolyte. Since little film is formed using this method we then monitored the changes in volume and morphology of patterned Sn electrodes as they reacted reversibly with lithium. The volume of the patterned tin electrodes grows with cycle number due to substantial morphology changes, unlike electrodes made from amorphous Si or amorphous Si-Sn. (C) 2003 The Electrochemical Society.