Hematoporphyrin IX, H ~2~ HMP , 8,13-bis(1-hydroxyethyl)-3,7,12,17-tetramethyl-21 H ,23 H -porphine-2,18-dipropionic acid and protoporphyrin IX, H ~2~ PP , 8,13-divinyl-3,7,12,17-tetramethyl-21 H ,23 H -porphine-2,18-dipropionic acid were efficiently immobilized on niobium oxide grafted on a silica gel surface, SiO ~2~/ Nb ~2~ O ~5~, by the - COO – Nb bond formed between the porphyrin carboxyl groups and the grafted Nb ~2~ O ~5~. These immobilized porphyrins, SiO ~2~/ Nb ~2~ O ~5~/ H ~2~ HMP and SiO ~2~/ Nb ~2~ O ~5~/ H ~2~ PP , were further reacted with Co (II) in dimethylformamide, resulting in SiO ~2~/ Nb ~2~ O ~5~/ CoHMP and SiO ~2~/ Nb ~2~ O ~5~/ CoPP metallated complexes. The UV-vis spectra of the solid materials showed changes of the Q-bands ( a ~2 u~ → e ~g~ transition) upon metallation, indicating that by incorporation of Co (II) in the porphyrin ring the local symmetry changed from D~2 h~ to D~4 h~ . These materials, when incorporated in carbon paste electrodes, presented the property of electrocatalyzing O ~2~ reduction. Rotating disk experiments were performed in order to estimate the number of electrons involved in the process. It was observed that, for both modified electrodes, O ~2~ was reduced to water in a four-electron process. Amperometric studies showed the potentiality of both modified electrodes as sensors for the determination of dissolved dioxygen. The response time was less than 3 s. A linear response for both systems was obtained between 2 and 12 ppm.