In the present study, the thermal diffusivity and conductivity of WE54 and Elektron 21 alloys were studied. The results showed the thermal diffusivity of WE54 and Elektron 21 alloys were temperature and microstructure dependent. The thermal diffusivity of both alloys was dependent on the content of the solute element in the α-Mg matrix. The solid solution of Y and Gd in Mg has a lower thermal conductivity than alloys where the intermetallic Mg3(Nd,Gd) and Mg14Y2Nd phases are present. The formation of strengthening phases during ageing caused the consumption of the solute element in the α -Mg matrix, and improved the thermal conductivity of the alloys.