Carboxylic multi-wall carbon nanotubes (MWCNT) were directly cast onto the glassy carbon electrode to fabricate the MWCNT modified electrode, which showed good stability and reproducibility. The modified electrode exhibited good promotion to the electrochemical reaction of theophylline (TP) and greatly enhanced the peak currents. The possible mechanism of the catalytic oxidation of TP was investigated by means of cyclic voltammetry and UV-Vis spectroscopy. Under optimal conditions there was a good linear relationship between anodic peak current and TP concentration in the range from 3 x 10(-7) to 1 X 10(-5) mol L-1, and a detection limit of 5 x 10(-8) mol L-1 (S/N = 3) was achieved after 2 min of open-circuit accumulation. The MWCNT modified electrode can be applied to determination of TP in drug. (c) 2005 Elsevier B.V. All rights reserved.