The absence of Mg-ion trapping in selenium-based Chevrel phase (in contrast to charge trapping in Mg x Mo 6 S 8 ) was carefully monitored using a combination of three electroanalytical tools: SSCV, PITT and EIS. We discuss the difference between Mg x Mo 6 S 8 and Mg x Mo 6 Se 8 (0<x<2) in terms of the higher polarizability of the host's Se 2− anions framework, compared to that of S 2− , in sulfur-based Chevrel phase, which presents lower diffusion barrier for the mobile Mg ions. In addition, it is shown that the formal equilibrium potential of Mg ions insertion is higher for Mo 6 S 8 compared to that of Mo 6 Se 8 . This difference relates also to the higher stability of the selenium anions framework compared to the sulfur-based anionic system.