Understanding the mechanism and thermodynamics of the adsorption of chemicals on carbon nanotubes (CNTs) is important to risk assessment and pollution control of both CNTs and chemicals. We computed the adsorption of cyclohexane, benzene derivatives, and polycyclic aromatic hydrocarbons (PAHs) on (8,0) single-walled carbon nanotubes by the M05-2X of density functional theory. The computed adsorption energies (E(a)) in the aqueous phase are lower than those in the gaseous phase, indicating that the adsorption in the aqueous phase is more favorable. The contribution of π-π interactions and the enhancing effect of a -NO(2) substituent on the adsorption were quantified. For a hypothetical aromatic with the same hydrophobicity (logK(OW)) to cyclohexane, π-π interactions contribute ca. 24% of the total interactions as indicated by E(a). -NO(2) enhances the π-π interactions due to its electron withdrawing effects, and contributes 24% to the value of E(a). Simple linear regression showed the computed Gibbs free energy changes for the adsorption correlate significantly with the experimental values (r = 0.97, p < 0.01). The correlation together with the computed thermodynamic parameters may be employed to predict the adsorption affinity of other chemicals. The study may pave a new way for evaluating/predicting the adsorption affinity of organic compounds on SWNTs and probing the adsorption mechanisms.