The development of a highly sensitive sensor for oxygen is proposed using a glassy carbon (GC) electrode modified with alternated layers of iron(II) tetrasulfonated phthalocyanine (FeTsPc) and iron(III) tetra- (N-methyl-pyridyl)-porphyrin (FeT4MPyP). The modified electrode showed excellent catalytic activity for the oxygen reduction. The reduction potential of the oxygen was shifted about 330mV toward less negative values with this modified electrode, presenting a peak current much higher than those observed on a bare GC electrode. Cyclic voltammetry and rotating disk electrode (RDE) experiments indicated that the oxygen reduction reaction involves 4 electrons with a heterogenous rate constant (K-obs) Of 3 x 10(5) mol(-1) Ls(-1). A linear response range from 0.2 up to 6.4 mg L-1, with a sensitivity of 4.12 mu AL mg(-1) (or 20.65 mu A cm(-2) Lmg(-1)) and a detection limit of 0.06 mg L-1 were obtained with this sensor. The repeatability of the proposed sensor, evaluated in terms of relative standard deviation (R.S.D.) was 2.0% for 10 measurements of a solution of 6.4 mgL(-1) oxygen. The sensor was applied to determine oxygen in pond and tap water samples showing to be a promising tool for this purpose. (C) 2008 Elsevier B.V All rights reserved.61212936