© 2015 The Electrochemical Society. This work focuses on the effect of multiwall carbon nanotubes (MWCNTs) purification methods for their application as conductive materials in the development of MWCNTs/epoxy amperometric nanocomposite (bio)sensors. For this purpose, three different MWCNTs samples with distinctive purities were characterized by Termogravimetric Analysis, X-Ray Fluorescence Spectroscopy and Transmission Electron Microscopy. Subsequently, the samples were used to fabricate three different series of MWCNTs dispersed into resin epoxy. These series contained from 1% to 12% of the MWCNT sample. Composition ratios were modelled by percolation theory and characterized by different electrochemical techniques including Cyclic Voltammetry and Electrochemical Impedance Spectroscopy. After accurate electrical and electrochemical characterization, it has been demonstrated that the purification method affects the electrochemical behavior of the nanocomposite electrodes; however the optimum MWCNT/epoxy ratio was not modified. Furthermore, morphological experiments corroborated that the electrochemical performance of the electrodes closely depends on the physical properties of the different MWCNTs used. The optimized-sensors (near-percolation sensors) were tested by hydrodynamic amperometry, using ascorbic acid as a model analyte. Interestingly, the sensors containing non-purified MWCNTs exhibited the best electroanalytical response. This fact demonstrates the beneficial effects of metal impurities being present in MWCNTs to enhance the analytical response of MWCNT-based amperometric nanocomposite (bio)sensors.