We present the magnetic, optical and relaxometric properties of multifunctional Au-Fe3O4 hybrid nanoparticles (HNPs), as possible novel contrast agents (CAs) for magnetic resonance imaging (MRI). The HNPs have been synthesized by wet chemical methods in heterodimer and core-shell geometries and capped with oleylamine. Structural characterization of the samples have been made by X-ray diffraction and transmission electron microscopy, while magnetic properties have been investigated by means of Superconducting Quantum Interference Device-SQUID magnetometry experiments. As required for MRI applications using negative CAs, the samples resulted superparamagnetic at room temperature and well above their blocking temperatures. Optical properties have been investigated by analyzing the optical absorbtion spectra collected in UV-visible region. Relaxometric measurements have been performed on organic suspensions of HNPs and Nuclear Magnetic Resonance (NMR) dispersion curves have been obtained by measuring the longitudinal 1/T-1 and transverse 1/T-2 relaxation rates of solvent protons in the range 10 kHz1300 MHz at room temperature. NMR relaxivities r(1) and r(2) have been compared with ENDOREM (R), one of the commercial superparamagnetic iron oxide based MRI contrast agents. MRI contrast enhancement efficiencies have been investigated also by examining T-2-weighted MR images of suspensions. The experimental results suggest that the nanoparticles' suspensions are good candidates as negative CAs. (C) 2012 Elsevier B.V. All rights reserved.