Tetra-n-butylammonium (TBA) salts of the cobalt-monosubstituted Keggin polyoxometalate (Co-POM), TBA(4)HPW(11)CoO(39) (I) and TBA(5)PW(11)CoO(39) (II), have been prepared and characterized by using elemental analysis, potentiometric titration, IR, UV-vis, P-31 NMR, and cyclic voltammetry. Different modes of heterogenization of the Co-POM, including anchoring to both NH2- and NH3+-modified mesoporous silica surfaces and sol-gel synthesis, have been performed. The resulting solid catalysts were characterized by N-2-adsorption measurements, elemental analysis, DR-UV-vis, and FT-IR spectroscopy. The activity of the solid Co-POM materials to catalyze aerobic oxidation of two representative aldehydes, isobutyraldehyde (IBA) in MeCN and formaldehyde in H2O under mild conditions (20-10 degreesC, 1 atm of air), was assessed and compared with the catalytic activity of the corresponding homogeneous Co-POMs. An emphasis has been placed on leaching tests and catalyst recycling. The effect of protonation of the amine-modified silica surface or the Co-POM on the loading, stability, and activity of the Co-POM solid catalyst was evaluated. (C) 2004 Elsevier Inc. All rights reserved.