The dissociation of SO3 molecules on the walls of armchair (m, m) (m = 3, 5) and zigzag (n, 0) (n = 5-10) single-wall carbon nanotubes (SWCNTs) was investigated using first-principles calculations. The calculated reaction enthalpies and energy barriers show a nearly linear decrease with decreasing tube diameter. Our results indicate that the dissociation of SO3 on the walls of SWCNTs is strongly dependent on the tube diameter for the selective etching of SWCNTs with small diameters (d < 1 nm) by gaseous SO3. Our study sheds light on effective separation of SWCNTs for their practical application in many fields. (c) 2014 Elsevier B.V. All rights reserved.