A conductive composite film which is composed of functionalized multiwalled carbon nanotubes (f-MWCNTs) with poly(neutral red) (PNR) has been synthesized using the potentiostatic method. It is produced on glassy carbon, gold and transparent semiconductor tin oxide electrodes for electrocatalysis, electrochemical quartz crystal microbalance (EQCM), and scanning electron microscopy (SEM) studies. The incorporation of PNR on the f-MWCNTs is revealed by the EQCM technique and their morphology has been studied using the SEM method. The composite film exhibits a promising electrocatalytic activity toward biochemical compounds (in pH 6.0), halogen oxyanions (pH 1.0), and sulfur oxoanions (pH 6.0). Among these, the biochemical compounds which have been studied are (-)-epigallocatechin gallate, nicotinamide adenine dinucleotide, epinephrine, and serotonin. The sulfur oxoanions are S2O42-, SO32-, S2O82-, and SO52-. The presence of f-MWCNTs amplifies the electrocatalytic peak current and the film background current. The results confirm also that the f-MWCNTs enhance electrocatalytic oxidation and PNR enhances reduction. Further, the halogen oxyanions electrocatalytic reduction potential Ep(c) for IO3-=-0.25 V, BrO3-=-0.5 V and no peak for ClO3- clearly reveal that the activity is dependent on its electronegativity, which is one of the basic science properties. (c) 2007 The Electrochemical Society.