When polyelectrolyte-neutral block copolymers are mixed in aqueous solutions with oppositely charged species such as surfactant micelles, multivalent counterions or inorganic nanoparticles, there is the formation of stable aggregates, also called colloidal complexes or coacervates. The properties of these complexes are: i) fast and spontaneous aggregation by compensation of opposite charges; ii) core-shell type microstructure, and in some cases monodispersity in size. From this microstructure, these colloids resemble the well-known amphiphilic block copolymer micelles. In this contribution, we study the complexation mechanism between neutral-polyelectrolyte block copolymers with inorganic nanoparticles made of a superparamagnetic oxide (maghemite gamma-Fe2O3), with diameters around 6 nm. The objective is to provide optimized contrast agents for Magnetic Resonance Imaging (MRI). Protected by the neutral polymer corona, the about 100 nm iron oxide-copolymer complexes could also be used as stealth particles in living organisms, i.e. vectors for drug delivery able to delay phagocytosis. The copolymer used here is abbreviated PTEA-b-PAM: the catanionic polyelectrolyte block is a poly(trimethylammonium ethylacrylate methylsulfate) chain and the neutral block is poly(acrylamide). The Fe2O3–nanoparticles have their surfaces coated by citrate ligands which are negatively charged at neutral pH. The polymer-nanoparticles complexes are obtained by mixing pure solutions prepared at the same concentration c and pH 7. The mixing ratio X is defined as the volume of iron oxide sols relative to that of the polymer. For all the mixed solutions, no phase separation or precipitation have been observed during or after the mixing. We follow the formation of mixed nanoparticle-polymer complexes by Static Light Scattering measurements at 90°. The scattered intensity I90°(X) passes through a maximum at XP ~ 1 for all studied mixtures. The number of nanoparticles N per mixed aggregate was given by fitting these curve I90°(X): N was found equal to 10, in accordance with the analysis of pictures made by Cryo-TEM on those samples. Dynamic Light Scattering performed on the solutions confirms the hypothesis of the formation of mixed aggregates: for PTEA(11k)-b-PAM(30k), DH is of the order of 100 nm well above those of the individual components and it decreases slightly for PTEA(5k)-b-PAM(30k). Finally, Magnetic Resonance Spin-Echo measurements we performed to measure the increase of the ratio between transverse and longitudinal relaxivities of protons R2/R1 caused by the superparamagnetic nanoparticles. The mixed aggregates were found to have a higher R2/R1 ratio than individual nanoparticles. These results suggest that the polymer-nanoparticle aggregates could be used as T2 contrast agents.