A study of modified carbon paste electrode employing Nile blue (NB) adsorbed on silica gel modified with niobium oxide (SN) for electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH) is described. The adsorbed organic dye on SN was used to prepare a modified carbon paste electrode to investigate its electrochemical properties. The formal potential (E-o') of the adsorbed NB (-230 mV vs. saturated calomel electrode, SCE) showed a shift of 70 mV towards a more positive potential value, compared to NB dissolved in aqueous solution. In solutions with pH between 6.0 and 8.0 did stability and E-o' remained almost constant. However, for a solution pH lower than 6.0 the E-o' was affected by the acidity of the contacting solution, shifting the E-o' towards more positive values. For the solution pHs between 6.0 and 8.0 the electrocatalytic activity remained almost constant. A linear response range for NADH between 1.0 x 10(-5) and 5.2 x 10(-4) mol 1-1, at pH 7.0, was observed for the electrode, with an applied potential of -200 mV versus SCE. The formation of an intermediate charge transfer (CT) complex was proposed to the CT reaction between NADH and adsorbed NB. The heterogeneous electron transfer rate, k(obs), was 1400 M-1 s(-1) and the apparent Michaelis-Menten constant, was 0.21 mM at pH 7.0 evaluated from rotating disk electrode (RDE) experiments with an electrode coverage of about 5.2 x 10(-9) mol cm(-2). The increase in the reaction rate between NADH and the immobilized NB compared to those obtained with dissolved NB was assigned to the shift of the E-o' towards more positive values.