Two new kinds of electrochemical sensors were prepared using glassy carbon electrodes modified with poly(9-aminoacridine) (PAA) and PAA-functionalized multi-walled carbon nanotubes (PAA-MWCNTs) for the determination of tryptophan (Trp) in the presence of tyrosine (Tyr). The electrocatalytic response of the modified electrodes towards electrooxidation of Trp and Tyr was investigated and compared by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The surface of the modified electrodes was characterized using CV, scanning electron microscopy, scanning-tunneling microscopy and scanning-tunneling spectroscopy. The anodic peak current of the electrooxidation of Trp on PAA-MWCNTs electrode was 5.5-fold increased comparing to PAA modified electrode. The anodic peak potential of Trp is shifted about 0.12 V to more negative value indicated that PAA-MWCNTs electrode has better electrocatalytic activity for electrooxidation of Trp. Furthermore, the anodic peak current which was attributed to the electrooxidation of Trp did almost not change with the addition of Tyr in buffer solution at pH 3.5. Besides this, at pH 7.4, the anodic peak current obtained at 0.6V increased as the concentration of Tyr was increased. The results demonstrate that PAA-MWCNTs electrode can be used for the simultaneous determination of Trp and Tyr by adjusting the pH of the solution. (C) 2011 Elsevier Ltd. All rights reserved.