Tin anodes show a rich structure and reaction chemistry which we have investigated in detail. Upon discharge five plateaus are observed corresponding to β-Sn, an unidentified phase (Na/Sn = 0.6), an amorphous phase (Na/Sn = 1.2), a hexagonal R-3m Na5Sn2, and fully sodiated I-43d Na15Sn4. With charging there are six plateaus related to the formation of Na5Sn2 followed by the formation of amorphous phases and β-Sn. Upon cycling the formation of metastable Na5Sn2 seems to be suppressed. Theoretical voltages calculated from existing crystal structures using DFT provide a good match with constant current and quasi-equilibrium measurements (GITT). Search for additional (meta)stable phases using cluster-expansion method predicts many phases lower in energy than the convex hull obtained from known structures, including the R-3m Na5Sn2 phase. The presence of multiple phases in varying lattices with similar formation energy suggests why the reaction mechanism is non-reversible. 119Sn Mössbauer spectroscopy results indicate a decrease of the isomer shift with increasing Na/Sn content, which is less pronounced than for Li–Sn compounds likely due to the lower electropositivity of Na. The electrode surface is terminated with an SEI layer rich in carbonates (Na2CO3 and Na CO3R) as evidenced by XPS. After charge at 2 V, strong evidence for the formation of oxidized Sn4+ is obtained. Subjecting the electrode to a rest after charge at 2 V reveals that aging in the electrolyte reduces the oxidized Sn4+ into Sn2+ and Sn0, and concomitantly suppresses the electrolyte decomposition represented by an anomalous discharge plateau at 1.2 V. Thereby, the catalytic decomposition of the electrolyte during discharge is caused by nanosized Sn particles covered by oxidized Sn4+ and not by pure metallic Sn.