Mesostructured V-Mg oxides were synthesized using the surfactant cetyltrimethylammonium. bromide (CTAB) as template, V2O5, V(acac)(3) (vanadium acetylacetonate), or NH4VO3 as vanadium source, and Mg(NO3)(2), MgCl2, MgSO4, (MgCO3)(4.)Mg(OH)(2), Mg(CH3CO2)(2), or Mg(C2H5O)(2) as magnesium source. The factors that influence the formation of mesostructured V-Mg oxides, such as the pH, the natures of magnesium and vanadium sources, and the ionic strength, were identified. The formation of mesophases could be related to the presence of anionic vanadium species, to the electrostatic interactions between the oppositely charged vanadates and micellar headgroups, and to the nature of the counterion of Mg2+ in the magnesium source. The main role was played by the pH and only when the pH allowed the formation of vanadates was a mesostructure generated. The counterions of Mg2+ also played a role, which could be explained via specific ion effects and the formation of complexes between them and the vanadium-containing species, which are attracted by the headgroups of the micellar templates.