Grain refiners for Al-containing Mg-alloys are still deficient and under investigation. To clarify the mechanisms of grain refinement by inoculation a simulation method for heterogeneous nucleation has been used, allowing the prediction of the grain size for a given particle size distribution, volumetric content of inoculants, cooling rate and alloy constitution. To proof the model, grain refinement efficiency of (SiC)_P in Mg3wt%Al was examined experimentally. It was found that (SiC)_P results in significant grain refinement. Experiments with (SiC)_p in Al-free Mg-alloys have shown that additionally to the low lattice misfit of SiC, solutes play an important role for effective grain refinement. While the grain refinement via (SiC) _p inoculation of pure Mg is negligible, a distinct effect has been achieved in Mg3wt%Zn. The model makes qualitatively correct predictions for the grain size and its variation with cooling rate. Relevant aspects for the consideration of particle-specific properties and solutes will be discussed.