Inverse gas chromatography (IGC) was used to characterize the surface properties of pristine multi-walled carbon nanotubes (MWNTs), as well as the poly(acrylic acid) sidewall covalently functionalized MWNTs (PAA-g-MWNTs) and hydroxyl group directly grafted MWNTs (MWNTols). The dispersive component of the surface energy (gamma(D)(S)) and the acid/base character of these samples' surfaces were estimated by the retention time with different non-polar and polar probes at infinite dilution region. The specific free energy (Delta G(AB)) and the enthalpy (Delta H-AB) of adsorption corresponding to acid-base surface interactions were determined. By correlating Delta H-AB with the donor and acceptor numbers of the probes, the acidic (K-A) and the basic K-D parameters of the samples' were calculated. The results show that chemical modification successfully reduces the dispersive component of the surface energy of MWNTs. Furthermore, MWNTs grafted with hydroxyl groups exhibit a more basic character, while MWNTs grafted with poly(acrylic acid) show a more acidic character. Overall, IGC provides useful complementary information on the changes resulted from the chemical modifications of the surface.