An effective, sensitive and simple method as a direct electroiodimetric determination of reducing biological substances using a modified multiwall carbon nanotube/ionic liquid paste electrode has been introduced. Tetra-n-octylammonium triiodide (TOAI(3)) was used as a new modifier which was simply synthesized and characterized. By combining the advantages of both cyclic voltammetry and iodimetric technique, the problems associated with the classical iodimetry such as time consuming stages and indicator usage were removed. Three types of carbon paste electrodes (CPEs) with different structures were studied and their compositions were optimized. The appropriate one which was MWCNT/TOAI(3)/CILE (multiwall carbon nanotube/tetra-n-octylammonium triiodide/carbon ionic liquid electrode), displayed unique electrocatalytic behavior in acidic media and was validated for determination of ascorbic acid (AA), cysteine (Cys) and dopamine (DA) as typical reducing biomolecules in pharmaceutical and human blood serum samples. By applying cyclic voltammetry, the linear response ranges for the determination of AA, Cys and DA were recorded as 1.0-2500.0 mu M,1.0-1500.0 mu M, and 0.8-800.0 mu M, in order. The theoretical detection limits were found to be 1.0, 0.8, and 0.6 mu M for the determination of AA, Cys, and DA, respectively. The effect of some coexisting biomolecules in determination of these analytes was investigated and glucose effectively interfered. (C) 2018 Elsevier Ltd. All rights reserved.