The interactions of a series of novel cationic gemini surfactants, [CnH2n+1(CH3)(2)N+CH2CHOHCHOHCH2(CH3)(2)N+CnH2n+1](2) 2Br(-) (symbolized (CnN)(2)), with conventional surfactants (containing a single hydrophilic and a single hydrophobic group in the molecule) have been investigated. It is found that (C8N)(2) and (C10N)(2) have very strong interactions with the anionic surfactants C10H21SO3Na, C12H25SO3Na, and C12H25SO3Na, and C12H25(C2H4O)(4)SO4Na, producing marked synergism in surface tension reduction efficiency and effectiveness and in mixed micelle formation in H2O, 0.1 M NaBr, and 0.1 M NaCl. In contrast to this, (C12N)(2) shows no synergism at all with C12H25SO3Na and has a much weaker synergistic interaction with C12H25(C2H4O)(4)SO4Na It is proposed that extraordinarily strong interaction between the (C12N)(2) and C12H25SO3Na produces small, soluble, nonmicellar aggregates having no surface activity that decrease the monomer concentrations of the component surfactants, thereby reducing the surface activity of the system. Equilibrium constant calculations for this aggregate formation indicate that the two surfactants are present in a 1:1 molar ratio.