The tie lines between (CoXMg1−X)O solid solution with rock salt structure and orthosilicate solid solution (CoYMg1−Y)‐Si0.5O2, and between orthosilicate and metasilicate (CoZMg1‐Z)SiO3crystalline solutions, have been determined experimentally at 1373 K. The compositions of coexisting phases have been determined by electron probe microanalysis (EPMA) and lattice parameter measurement on equilibrated samples. The metasilicate solid solution exists only for 0>Z>0.213. The activity of CoO in the rock salt solid solution was determined as a function of composition and temperature in the range of 1023 to 1373 K using a solid‐state galvanic cell:Pt, (CoXMg1−X)O+Co|(Y2O3)ZrO2|Co+CoO, PtThe free energy of mixing of (CoXMg1−X)O crystalline solution can be expressed by the equationΔGE=X(1 −X)[(6048 − 2.146T)X+ (8745 − 3.09T)(1 −X)] J·mol−1The thermodynamic data for the rock salt phase is combined with information on interphase partitioning of Co and Mg to generate the mixing properties for the ortho‐ and metasilicate solid solutions. For the orthosilicate solution (CoYMg1 −Y)Si0.5O2at 1373 K, the excess Gibbs free energy of mixing is given by the relationΔGE=Y(1 −Y)[2805Y+ 3261(1 −Y)] J·mol−1For the metasilicate solution (CoZMg1 −Z)SiO3at the same temperature, the excess free energy can be expressed by the relati