The Growth Restriction Factor, Q, introduced by David StJohn and his team, is the key quantity in current descriptions of the solutal effect on grain growth and grain refinement during solidification of alloys. A rigorous approach for the evaluation of Q in multicomponent alloys requires consistent thermodynamic descriptions of the pertinent alloy phase equilibria. Grain refinement of Mg-Al based alloys is still challenging because it is known that Zr, which is extremely effective in many Al-free alloys, cannot be used. The addition of carbon through various routes by using carbon-containing sources is considered as an option. The present work will focus on current, ongoing work on thermodynamics of carbon and carbides in selected multicomponent Mg-alloys. The competition between strong and weak carbide-forming alloying components will be highlighted, for a better understanding of grain refinement in such Mg-alloys.