The surface conductivity of colloidal nanotubes, induced by ionic surfactants, is known to affect alternating current dielectrophoresis, which has been actively investigated with regard to separating single-walled carbon nanotubes according to electronic type. The nanotube-to-suspending medium conductivity ratio is a primary factor for determining the dielectrophoretic behavior of semiconducting nanotubes. In this study, our theoretical and experimental analysis revealed that the suspending medium conductivity also plays an important role in controlling the conductivity ratio. This work elucidates the effects of several surfactant systems on the conductivity ratio and therefore the degree of separation between metallic and semiconducting nanotubes. The equimolar mixture of anionic and cationic surfactants was more effective than a nonionic polymer in reducing the conductivity ratio because the conductivity of colloidal nanotubes was decreased and that of the suspending medium was increased. Besides, the surfactant mixture provided a better dispersion of nanotubes. The dielectrophoretic separation was carried out using microelectrodes with a gap size of 4 mum at an electric field frequency of 10 MHz. The complete separation of nanotubes at the reduced conductivity ratio was confirmed by Raman spectroscopy and electrical transport measurements.