Abstract Carrier transport in Mg 2 Si 1-x Sn x thermoelectrics was experimentally found to be highly sensitive to high temperature heat treatment and in particular the role of the precise Mg content has been discussed controversially. Considering this, electrical transport properties of Sb doped Mg 2 Si 0.4 Sn 0.6 were measured in-situ during annealing at 710 K. We measured two quasi-identical samples: sample 1, for the Seebeck coefficient and the electrical conductivity (σ) measurement in helium, and sample 2 for Hall coefficient and σ measurement in vacuum, respectively. Both samples largely remain single phase and did not show de-mixing after annealing for∼ 276 hours and∼ 1100 hours respectively. The observed experimental data can be modelled using a single-and two parabolic band model and a continuous reduction in majority carriers is identified, which can be linked to Mg loss. Thorough analysis furthermore reveals mobility loss and a lowering of the density of states effective mass which could both be due to ongoing Mg loss or due to a lifted degeneracy of conduction bands in Mg 2 Si 0.4 Sn 0.6 at room temperature. Finally, we can link the change in carrier concentration to a change in Mg-related defects and identify a phase width Δ δ in Mg 2-δ Si 0.4 Sn 0.6.