Competitive adsorption between nonpolar organic compounds and polar ionic organic compounds (IOCs) on carbon nanotubes (CNTs) is essential for application of CNTs as superior sorbents and for environmental risk assessment of both CNTs and organic contaminants. It was observed in this study that adsorption of neutral and dissociated species of polar 2,4-dichlorophenol (DCP) and 4-chloroaniline (PCAN) on a multiwalled CNT sample (MWCNT15) can be suppressed by nonpolar naphthalene. Naphthalene adsorption can also be suppressed by neutral DCP/PCAN, but not dissociated DCP/PCAN. Moreover, competition of naphthalene decreased the adsorption affinity of neutral DCP/PCAN, but not their adsorption capacity because of the formation of solute bilayer on MWCNT15. For dissociated DCP/PCAN, naphthalene not only decreased their adsorption affinity but also their adsorption capacity because no solute bilayer was formed. Neutral DCP/PCAN also decreased the adsorption affinity and adsorption capacity of naphthalene. These observations indicate that competitive adsorption of naphthalene with DCP/PCAN depends on the dissociation of DCP/PCAN, as interpreted by (i) the different sites on CNTs for adsorption of organic chemicals (i.e., naphthalene, and the neutral and dissociated species of DCP/PCAN), (ii) the interactions between organic chemicals, and (iii) the interactions of organic chemicals with CNT surface.