This is the first study in the literature, which presents the voltammetric determination of herbicide clopyralid (CLD). A novel modified glassy carbon electrode was developed by using the nanocomposite containing acid-activated multi-walled carbon nanotube (A-MWCNT) and fumed silica (FS). The effective surface area of the electrode (8 times) and the reduction peak current values of CLP (369 times) increased in the presence of A-MWCNT-FS nanocomposite. This revealed that the electrode had an electro-catalytic effect on the electrochemical reduction of CLP. Cyclic voltammetry and electrochemical impedance measurements indicated that electron transfer rate of Fe(CN)(6)(4-/3-) redox couple on the modified electrode was much higher over the bare electrode. Scanning electron microscopy images showed that A-MWCNT and FS formed a homogeneous nanocomposite by dispersing each other. It was observed that electrochemical reduction of CLP was more favourable in pH values between 2 and 4. Electrochemical reduction of CLP on the modified electrode was an adsorption-controlled process. Electrochemical reduction peak currents linearly increased with CLP concentrations in the range of 0.005 mu M and 10.0 mu M. The detection limit was calculated as 0.8 nM. Measurement and fabrication reproducibilities of the modified electrode were obtained as 2.94% (N: 10) and 3.34% (N: 5), respectively. Electrochemical reduction behaviour of CLP was not affected by the presence of different substances in the measurement solution. The GC/AMWCNT-FS electrode allowed to determine the CLP in real samples such as urine, river water, sugar beet, wheat and herbicide formulations (Phaeton and Lontrel). The results indicated that the electrochemical sensor prepared in this study could be a very promising alternative to current analytical methods used to determine CLP. (C) 2017 Elsevier B.V. All rights reserved.