Electropolymerization of brilliant cresyl blue (BCB) in ethaline deep eutectic solvent (DES) has been carried out. Poly(brilliant cresyl blue) (PBCB) films were formed by electrodeposition on carbon nanotube modified glassy carbon electrodes by potential cycling in ethaline (choline chloride thorn ethylene glycol) with the addition of different ionic species i.e. NO3-, SO42-, Cl-, ClO4-. The novel nanocomposite films were characterized by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The composition of the polymerization solution had a large influence on the morphology as well as on the electrochemical behaviour of the electrodeposited films. The effect of scan rate on BCB polymerization was also investigated and the PBCB film synthesized at 150 mV s(-1) in ethaline + NO3- presented a superior electrochemical performance. The PBCBEthaline-HNO3/MWCNT and PBCBaqueous/MWCNT nanocomposite films were used as electrode support for the enzymes glucose oxidase and tyrosinase for the biosensing of glucose and catechol, respectively and were more sensitive than the PBCB based electrodes prepared in aqueous solution. (c) 2019 Elsevier Ltd. All rights reserved.