Commercial carbon nanotubes have been oxidized with HNO3, H2O2 and air and they have been tested on the cyclization of citronellal. The materials have been characterized by N-2 adsorption desorption isotherms, transmission electron microscopy, X-ray diffraction, temperature programmed desorption, potentiometry titration and X-ray photoelectron spectroscopy. The HNO3-treated samples displayed the largest concentration of carboxylic acids which increased by increasing the oxidation time and by a previous ball-milling treatment. The carboxylic acid groups were found to be the active groups on the reaction and the activity increased by increasing its content. The selectivity to isopulegols was higher (98%) over the most active samples (activity 0.20-0.68 mmol/g min), which has been explained by their smaller pore size caused by the oxidation treatment. The stability of the most active CNT sample was also proved by recycling the catalyst in 3 cycles. (C) 2016 Elsevier B.V. All rights reserved.