Alloys have the potential to fill the void for a reliable, high-performing anode material for sodium ion batteries. Here we investigate for the first time CoSn2 and FeSn2 and their reaction with sodium. These tin alloys can achieve capacities of up to 680 mAh/g if fully exploited, almost double of the standard anode material used today. We elucidate the reaction mechanism of these two isostructural alloys by analogy to the well understood Sn-Na system, and demonstrate that the transition metal believed to be inactive by the community in fact strongly influences the reaction mechanism. (C) 2016 The Electrochemical Society. All rights reserved.