The superior electrochemical activity of multi-walled carbon nanotubes (MWCNTs) of shorter dimensions towards the oxidation of propionic acid derivative drugs, namely ibuprofen (IBF) and naproxen (NPX), is presented. A glassy-carbon electrode was modified with MWCNTs of different sizes (diameter x length: 100-170 nm x 59 pm and 6-9 nm x 5 mu m, corresponding to LD- and SD-MWCNTs, respectively, larger and shorter diameter). Cyclic voltammetry of the SD-MWCNT-modified electrode showed a 200-300 mV decrease in the overpotential of oxidation reactions of both drugs and an increase in current (2 to 3.5-fold) in comparison with the LD-MWCNT-modified and unmodified electrodes. Similarly, the amperometric determination of both drugs using the SDMWCNT-modified electrode presented an increase in sensitivity in comparison with LD-MWCNT-modified electrode (2 to 2.8-fold). Improved performance of the SD-MWCNT-modified electrode was in agreement with electrochemical impedance spectroscopy measurements. These results indicate the higher electrocatalytic activity of SD-MWCNTs, which can be explained by the increased defect density revealed by Raman spectroscopic measurements. Additionally, we report the first amperornetric method for IBF determination using the SD-MWCNT-modified electrode, with a linear range from 10 to 1000 mu mol L-1, detection limit of 1.9 mu mol L-1, precision of 4%, and accuracy attested by capillary electrophoresis analyses. (C) 2016 Elsevier B.V. All rights reserved.