Mixed micelles of the dimeric surfactant ethanediyl-1,2-bis(dodecyldimethylammonium bromide), referred to as 12-2-12, and of its corresponding monomer dodecyltrimethylammonium bromide (DTAB) have been studied by small angle neutron scattering using a contrast variation method, in order to obtain information on the localization of DTAB in the threadlike micelles present in the mixed solutions at low DTAB mole fraction. The two surfactants are found to mix intimately, with. no evidence for a preferential localization of the spherical micelle-forming DTAB at the hemispherical endcaps of the threadlike micelles. Increasing DTAB mole fraction results in a decrease of the overall rigidity and mean length of the micelles. An appropriate normalization of the scattering curves-shows that this decreasing rigidity weakens the correlations between the micelles and suggests that this phenomenon is linked to a transition from a solution with some orientational order to a disordered solution.