Abstract Indium thin films were evaluated as an anode material for Li-ion and Na-ion batteries (theoretical capacities of 1012 mAh g−1 for Li and 467 mAh g−1 for Na). XRD data reveal that several known Li–In phases (LiIn, Li3In2, LiIn2 and Li13In3) form providing 950 mAh g−1 reversible capacity. In contrast, the reaction with Na is severely limited (75–125 mAh g−1). XRD data of short-circuited cells (40 h at 65 °C) show the coexistence of NaIn, In, and an unknown NaxIn phase. In electrodes exhibit anomalous electrolyte decomposition characterized by large discharge plateaus at 1.4 V vs Li/Li+ and 0.9 V vs Na/Na+. The presence of 5 wt% fluoroethylene carbonate additive suppresses the occurrence of the electrolyte decomposition during the first cycle but does not necessarily prevent it upon further cycling. Prevention of the anomalous decomposition can be achieved by restricting the (dis)charge voltages, increasing the current or by using larger amounts of FEC. The native surface oxides (In2O3) are responsible for the pronounced electrolyte decomposition during the first cycle while other In3+ species are responsible during the subsequent cycles. We also show that indium electrodes can exhibit very high rate capability for both Li (100 C-rate) and Na (30 C-rate).