CE has received considerable attention as a highly promising technique for metal ion analysis. Significant advances in resolution have occurred when metal-complex formation is introduced into the separation system. A better understanding of the migration mechanism and the ionic or molecular properties that control the separation of free or complexed metal ions requires a universal approach to be developed for analyzing relationships between migration parameters and charge-to-size characteristics of metal species, whatever their chemical form, as well as electrophoretic system variables. To meet this demand, a number of migration models, which are based on a generally valid equation for electrophoretic mobility as a function of charge density, were derived and evaluated using numerous sets of experimental migration data taken from the literature or obtained in the author's laboratory. Consistent approximation results confirm the validity of generally observed mechanisms of CE in the case of partially complexed metal ions and precapillary-formed metal complexes. The modeling approach proposed also appears to provide reliable and convincing proof for the input of distinctive complexing interactions to separation selectivity in different CE systems.