The catalysis of the reduction of dioxygen to hydrogen peroxide by different substituted 9-(4-R-phenyl)-N-methylacridiniums (R = H, Cl, F) was studied in homogeneous solutions and in Nafion(R) gel-modified electrodes. The stability of the acridinyl radical has been greatly improved by working in mixed Nafion(R) gels containing dibutylsulphoxide and tri(butoxyethyl)phosphate. The modified electrodes obtained containing the 9-(-4-R-phenyl)-N-methylacridinium salts show very good hydrophobicity and good stability in water solutions containing high concentrations of acetic acid (0.1 M). These compounds were able to catalyse the reduction of oxygen with a remarkable efficiency and the homogeneous electron transfer rate constants have been measured and found to be in the range of 10(-6) M-1 s-1. It was also possible to estimate the preconcentration factor between water and the gel around 20-25.