The adsorption behavior of sodium dodecyl sulfate (SDS) was investigated on titanium dioxide with quaternary ammonium groups (XNm, m is the carbon number of the alkyl chain, 4-16) by measuring amount adsorbed, zeta potential, and dispersion stability. The amount of SDS adsorbed increased sharply at low SDS concentration, reached a constant level on XN4, XN8, and XN12, and gradually increased on XN16. However, the amount of SDS adsorbed on untreated titanium dioxide was significantly low compared with that on XNm. The suspensions of XNm created by adsorption of SDS showed a dispersion-flocculation-redispersion sequence, accompanying a change in the zeta potential from positive to negative for all XNm. The flocs formed could be transferred to a toluene phase by adding toluene to the XNm suspensions, indicating that the flocs of all XNm are hydrophobic regardless of different alkyl chain lengths. The order parameter of methyl 12-doxylstearate located in the SDS adsorbed layer estimated by electron spin resonance shows that the mobilities of the probe are not restricted in the SDS adsorbed layer for all XNm. Adsolubilization behavior of 2-naphthol in the SDS adsorbed layer on XNm was peculiar; the amount of 2-naphthol adsolubilized was insensitive to the SDS concentration and was almost the same as the amount of 2-naphthol adsorbed on XNm in the absence of SDS, where the amount of 2-naphthol adsorbed increased with increasing alkyl chain length of XNm. In the case of untreated titanium dioxide, the amount of 2-naphthol adsolubilized increased with the SDS concentration, reached a maximum, and then decreased. Thus, the adsolubilization behavior of 2-naphthol for XNm was significantly different from that for untreated titanium dioxide.