Abstract The growth restriction factor Q is the key quantity in current descriptions of the solutal effect on grain growth and grain refinement during solidification of alloys. A rigorous treatment for the evaluation of Q in multicomponent alloys based on consistent thermodynamic descriptions of the alloy phase equilibria is presented. On closer inspection the conventional approach to calculate Q in multicomponent alloys from liquidus gradient m i and partition coefficient k i must fail for a wide range of common alloys exhibiting minute amounts of primary crystallizing intermetallic phase, exemplified for Mg–Al–Mn and Al–Si–Ti alloys. The rigorous approach provides an extension of the applicability range of the concept of Q. The qualitative similarity of inoculant particles and primary intermetallic phases is verified by calculations for Al–Si–Ti–B alloys.