The main aim of present study was to enhance the adsorption capacity of Reactive Blue 19 (RB19) onto multi-walled carbon nanotubes (MWCNTs), pretreated in a multistep functionalization process. The functionalization procedure consisted of microwave irradiation followed by. sonication in dilute H2SO4. The properties of MWCNTs were investigated by X-ray diffraction, scanning electron microscopy, N-2 adsorption isotherms (Brunauer-Emmett-Teller surface area analysis), Raman spectroscopy and Fourier transform infrared spectroscopy. The effective addition of desired functional groups resulted in a considerable increase of dye removal efficiency and adsorption capacity. For pristine and functionalized MWCNTs, the maximum adsorption capacities were found to be 53.33 and 211.02 mg/g, respectively. In addition to the main aim of this research, a statistical/mathematical approach-response surface methodology-was utilized to simulate and determine the optimum conditions of RB19 removal by functionalized MWCNTs using three selected parameters (adsorbent dose, initial dye concentration and pH). High R-2-value (97.75%) and a good agreement between predicted R-2-value (89.11%) and adjusted R-2-value (95.72%) demonstrated an acceptable proportion of the experimental and predicted results. For maximum RB19 removal efficiency, eight optimum scenarios were also obtained and validated by further experiments. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.