Three gemini surfactants, in which two quaternary ammonium species (CH3(CH2)(7)N+(CH3)(2)) are linked at the polar headgroups by o-, m-, or p-phenylenedimethylene spacers, have been synthesized. The critical micelle concentration (cmc) of these surfactants in aqueous solutions was determined by electrical conductivity. Selective-decoupling C-13 NMR and H-1 NMR spectra were measured at various concentrations below and above the cmc. The selective-decoupling C-13 NMR results revealed that the specific rotational isomers about the CH2-aromatic carbon single bonds for the gemini surfactant having a m-phenylenedimethylene spacer are preferentially stabilized upon micellization, while for the gemini surfactant having an o-phenylenedimethylene spacer, the presence of only the conformation in which the aromatic ring is sandwiched between two n-octyl chains was confirmed. Furthermore, it was found that variation in the stacking pattern of the aromatic rings after micellization of the surfactants is reflected in the H-1 spectral features of the aromatic protons. Comparison is made with the conformations of bis(quaternaryammonium) bromides with flexible spacer chains ((CH2)(s)) and of the corresponding n-alkylammonium bromide monomeric surfactants.