The　viscosity　of　aqueous　micellar　solutions　depends　on　the　size　and　shape　of　the　aggregates　and　thus　can　be　adjusted　by　addition　of　another　surfactant　interacting　with　the　original　component,　which　alters　the　geometry　of　the　molecule-pair　consisting　of　two　surfactants　and　influences　strongly　the　size　and　shape　of　the　mixed　micelles.　Ethanediyl-α,ω-bis(dimethyl　dodecyl　ammonium　bromide),　referred　to　as　C12-2-C12·2Br,　forms　generally　large　micelles.　Addition　of　a　cationic　surfactant　(dodecyltrimethylammonium　bromide,　C12TMABr)　or　a　nonionic　surfactant　(alkyl　polyoxyethylene　ether,　CmEn),　the　mixed　micelle　size　is　reduced　violently　since　the　electrostatic　repulsion　between　the　same　charged　heads　of　C12-2-C12·2Br　and　C　12TMABr　or　the　steric　hindrance　of　the　PEO　chain　of　C　mEn　in　the　palisade　layers　of　the　mixed　micelle,　which　leads　to　a　decrease　in　the　packing　parameter　P　of　the　molecule-pair.　As　a　result,　the　zero-shear　viscosity　(η0)　of　the　mixed　solution　reduces　rapidly.　In　contrast,　on　adding　an　oppositely　charged　surfactant,　η0　of　the　mixed　solution　increases　strongly　since　the　P　of　the　molecule-pair　increases　through　electrostatic　attraction　between　the　oppositely　charged　heads.　The　typical　cases　occur　in　the　mixtures　of　the　anionic　gemini　surfactant,　O,O＇　-bis(sodium　2-lauricate)-p-benzenediol　C11pPHCNa,　and　the　cationic　surfactant　C12-2-C12·2Br,　C　12TMABr　or　its　homologue　with　a　different　size　of　heads.　Copyright　©　2009　AOCS.