Mesoporous In2O3-y materials have been implanted using Co ions to induce a moderate ferromagnetic response at room temperature, forming a "mesoporous oxide-diluted magnetic semiconductor" (MODMS). X-ray photoemission spectroscopy (XPS) reveals that implantation results in up to 1 at. % Co (for 6 × 1015 ions/cm2 at 40 keV) and 15 at. % Co (for 1 × 1017 ions/cm2 at 60 keV). This is in both cases accompanied by a pronounced increase in the amount of oxygen vacancies with respect to the pristine, nonimplanted, In2O 3-y. Further increase in the ion fluence (up to 2 × 10 17 ions/cm2 at 60 keV) results in the collapse of the mesoporous structure, i.e., loss of the 3D-ordered porous configuration. XPS also reveals that virtually no metallic Co is formed at 40 keV, while a mixture of Co2+ and Co0 states is detected after implantation at 60 keV. Most of the Co2+ is incorporated in the bixbyite structure of the In2O3-y matrix. These results are consistent with previous models suggesting that the origin of the obtained ferromagnetic response in oxide-diluted magnetic semiconductors can be ascribed to ferromagnetic exchange interactions mediated by oxygen vacancies. This work constitutes the first report on MODMS prepared by nanocasting followed by implantation of transition metal ions. © 2013 American Chemical Society.