A study of a modified carbon paste electrode employing o-phenylenediamine (PDA) adsorbed onto silica gel modified with niobium oxide (SN) for electrocatalytic oxidation of nicotinamide adenine dinucleotide (NADH) is described. The species adsorbed on SN was used to prepare a modified carbon paste electrode to investigate its electrochemical properties. The formal potential (e) of the adsorbed PDA was -140 mV vs. SCE (saturated calomel electrode). The electrochemical behavior of the adsorbed PDA, compared to that of PDA dissolved in aqueous solution, was completely different. In solution, pH between 3.0 and 8.0, E-o' remained almost constant and the response was very stable. A linear response range for NADH between 4.0 x 10(-5) and 8.0 x 10(-4) Mol 1(-1), at pH 7.0, was observed for the electrode, with an applied potential of - 50 mV vs. SCE. The formation of an intermediate charge transfer complex is proposed for the charge transfer reaction between NADH and adsorbed PDA. The heterogeneous electron transfer rate, k(obs), was 5480 mol (-1) 1 s(-1) and the apparent Michaelis-Menten constant, 1.04 x 10(-4) Mol 1(-1) at pH 7.0, evaluated with rotating disk electrode (RDE) experiments with an electrode with a coverage of PDA of 5.7 x 10(-9) Mol cm(-2). The slight increase in the reaction rate with the solution pH was assigned to the thermodynamic driving force. (C) 2003 Elsevier Ltd. All rights reserved.