The electrochemical reactions of amorphous/nanocrystalline AlSb thin films prepared by magnetron sputtering are reported for the first time. The reaction with Li proceeds with an average reaction potential of 0.65 V, a reversible capacity of 750 mAh g−1, and very fast reaction kinetics. A storage capacity close to 500 mAh g−1 (70% of the maximum capacity) is achieved at 125 C-rate. Only small increases in overpotentials are measured with increasing currents: ∼0.15 V at 12 C and ∼0.7 V at 125 C. In contrast, the reaction with Na results in average reaction potential of 0.5 V and a storage capacity of 450 mAh g−1. The capacity retention and reaction kinetics with Na are presently not satisfactory with pronounced capacity losses upon cycling and larger overpotentials with increasing current. The capacity retention can be improved by using FEC additive in the Na-ion electrolyte, which highlights that the SEI plays an important role for the electrode cycling stability. The study of the reaction mechanism by X-ray diffraction reveals that the electrode material remains amorphous at all potentials, and suggests that during the reaction with Li the atomic short range ordering is similar to the expected phases.