The sodiation mechanism of Sn4P3 was studied by ex-situ X-ray diffraction and Mossbauer effect spectroscopy. These results show that Sn4P3 is initially amorphized during sodiation, and suggest that upon further sodiation both Sn and P react simultaneously with Na to form Na3P, Na0.6Sn, and Na4Sn4. Upon further sodiation, the Na-Sn phases react with Na to make Na5Sn2 and Na15Sn4. These results are consistent with the behavior of Sn4P3 in Li cells. (C) The Author(s) 2016. Published by ECS. All rights reserved.