The phase separation of the aqueous solutions of cationic-anionic surfactants, CsHI7 (OC2H4),,SO4Na CsH17N-(CH3)3Br (CsE.S CsNM3: n=0, 1. 3), CsS-CsH17 (OCeH4)3N{CH3)3C1 (CsE3NM3), CsE3S C,,.H_~,,,~IN{CH~)3Br (C,,NM3; m=10, 12), C12H25(OC2Ha),,SOnNa-CIeH25N(CH3)3Br {C12EnS-C12NM3: n=0, 3, 5, 7! and CsHIvCI, H 4 (OCeH4)~.sSO4NalCsqSE9.sS)-CmNM3 (m=12, 14, 16) has been investigated. The concentration region of phase separation [CRPS) of the aqueous solution of CsE,S-CsNM3 and C~2E,S-C~2NM 3 contracts with the increase in ll, and the CRPS disappears when n is equal to (or over 3) for CsE,S CsNM3 and 7 for C~zE.S C~eNM3. On the contrary, the CRPS of the aqueous solution of CsEsS C,,NM3, increases with the increase in m. The comparison between the molecular interaction parameter and the CRPS of the mixed system shows that the greater the molecular interaction between the two surfactant components, the easier it is for phase separation to occur, and vice versa. In the Csq5 E9.sS-C,,NM 3 system, there is no phase separation even if m is very large. The surface activity of a 1 : I cuqSEo.sS-CmNM3 mixture is still much higher than that of the single components. A relatively large number of oxyethylene (EO) groups introduced into the cationi~anionic surfactant molecule makes the mixed aqueous solution homogeneous at a very high concentration, which is of great significance in theory and practice.