The electrooxidation of ascorbic acid (AA), acetaminophen (AC), tryptophan (Trp), and their mixture has been studied using an 8,9-dihydroxy-7-methyl-12H-benzothiazolo[2,3-b]quinazolin-12-one modified carbon nanotubes paste electrode and cyclic voltammetry, chronoamperometry, electrochemical impedance spectroscopy and square wave voltammetry methods. The sensor exhibited potent and persistent electron mediating behavior followed by well separated oxidation peaks towards AA, AC and Trp with activation overpotential. The linear response range for AA, AC and Trp were 0.07–350, 3.0–600, and 10.0–700 μM, respectively. The modified electrode was used for the determination of these compounds in biological and pharmaceutical samples.