A detailed structural investigation on mixed micelle aggregates as target-specific contrast agents for magnetic resonance imaging technique has been carried out by way of small angle neutron scattering measurements. These mixed micelles are formed by two new amphiphilic molecules formed by a bioactive peptide and a claw moiety. The first molecule, C18H37CONH(AdOO)(x)-G-CCK8 (C18LxCCK8, x = 2, 5), contains an 18-carbon-atom alkylic chain bound to the C-terminal of the cholecystokinin octapeptide amide (CCK 26-33 or CCK8) and is able to bind to the overexpressed CCK receptor of some tumor cells. The second molecule, C(18)H(37)CONHLys(DTPAGlu)CONH25- (C18DTPAGlu) or its gadolinium complex [C(18)H(37)CONHLys(DTPAGlu)CONH2Gd](2-), C18DTPAGlu(Gd),contains the same 18-carbon-atom alkylic chain bound, through a lysine residue, to the DTPAGlu chelating agent. Small angle neutron scattering measurements have been performed on ternary systems at different total concentrations and at various ratios of the two molecules. The effect of the concentration on the aggregation number as well as on the shape of the micelle has been investigated. Furthermore in order to optimize the exposure of the peptide on the micelle surface, C18LxCCK8 having the spacer L of different length has been used. The pure binary systems of the synthesized molecules are also presented.