This study aims at the electrochemical characterization of a novel sensor for the electrocatalytic determination of morphine (MO). The sensor is based on a carbon paste electrode spiked with 4-hydroxy-2-(triphenylphosphonio)phenolate (HTP) and multi-wall carbon nanotubes (HTP-MWCNT-CPE). The cyclic voltammetric responses of MO oxidation at the modified electrode surface at different potential scan rates show a characteristic shape typical of an EC catalytic mechanism. Cyclic voltammetry, chronoamperometry, and differential pulse voltammetry (DPV) were used to probe the characteristics of the modified electrode. The catalytic peak current obtained by DPV was linearly dependent on the MO concentration over the range 1.0-950.0 mu M in two linear segments with a detection limit of 0.066 mu M. The precision of DPV was found to be 3% for 15 replicate determinations of 4.0 mu M of MO. For a binary mixture containing MO and acetaminophen (AC), two well-distinguished differential pulse voltammograms were obtained in the physiological pH (pH 7.0). The sensitivities of the modified electrode toward MO in the absence and presence of AC were found to be virtually the same, which refers to the fact that the electrocatalytic oxidation processes of MO are independent of AC. The modified electrode was successfully applied for the determination of MO and AC in a urine sample and pharmaceutical formulations. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.