Despite strong physical and chemical similarities between antimony and bismuth, a distinct behaviour is observed in the electrochemical magnesiation of their micrometric powders. Bismuth undergoes a complete and highly reversible alloying reaction, whereas antimony displays no electrochemical activity. Taking advantage of the complete SbBi solid solution, monophasic compositions Sb1-xBix were prepared by high-energy mechanochemical synthesis and characterized by X-ray diffraction and solid-state 25Mg nuclear magnetic resonance spectroscopy. The electrochemical magnesiation at low current rate shows a full alloying process of Sb1-xBix-based electrodes leading to monophasic Mg3(Sb1-xBix)2. This chemical association of antimony and bismuth enables a positive effect on the electrochemical magnesiation of the electrode and enables higher specific capacities compared to Bi-based electrodes. However, this synergy only operates in the nominal discharge since an irreversible capacity loss, which scales with the antimony content, is observed in the subsequent charge. Operando XRD reveals a complex segregation process leading to pure bismuth and Mg3Sb2 at the end of charge which is further rationalized by density functional theory calculations as an instability of the Mg3(Sb1-xBix)2 solid solution.