A Ce0.9Pr0.1O2 catalyst has been prepared using nitrates as metal precursors, which have been infiltrated into a polymethylmethacrylate colloidal crystal template and calcined at 500 degrees C. Its catalytic activity for CO oxidation (4.3 mu mol(CO) s(-1) g(catalyst)(-1) at 300 degrees C) is far superior compared to a reference catalyst prepared in the same way but without the template (1.4 mu mol(CO) s(-1) g(catalyst)(-1) at 300 degrees C), and even superior to a three dimensionally ordered macroporous (3DOM) catalyst prepared forming metal citrates to obtain the macroporous structure into the template (0.7 mu mol(CO) s(-1) g(catalyst)(-1) at 300 degrees C). It is demonstrated that infiltration of nitrates into the colloidal crystal template is more positive for catalytic oxidation of CO than formation of the 3DOM structure using citrates, despite the collapse of the macroporous structure. The 3DOM catalyst prepared with citrates has lower surface area (51 m(2) g(-1)) than catalysts prepared using nitrates (118 and 87 m(2) g(-1) for the infiltrated and non infiltrated catalysts, respectively). The infiltrated catalyst prepared with nitrates reaches the highest surface area, and also develops more micro and mesoporosity and presents smaller crystallite size (8 vs. 16 nm) than the counterpart non infiltrated catalyst. In addition, the population of oxygen vacancies is also much higher in the infiltrated catalyst prepared with nitrates, and it is postulated that this improves oxygen mobility into the mixed oxide lattice and favors the catalyst reduction. On the contrary, it has been ruled out any effect of the infiltration step in the insertion of Pr cations into the ceria lattice.