MRI contrast properties of self-aggregated nanoparticles made from GdIII and 1,2,4,5-tetrakis(pyrazol-1-yl-methyl-3-carboxylate)benzene (L) are investigated. Viscosity of GdL2 suspensions and size-characteristics of GdL2 nanoparticles allow an estimate of their large rotational correlation time. Moreover, two to three H2O molecules are bound, on average, to GdIII ions, as deduced from 17O NMR ... [Show full abstract] titration of the DyIII analogue. The large relaxivity of the particles, along with the prominent peak in the range 20–60MHz, are the consequence of these two properties. Longitudinal (r1) and transverse (r2) relaxivities are determined as a function of monomer concentration at 20°C and 20MHz. The ratio r1/r2 appears to be favorable for MR imaging using T1-weighted gradient echo sequences. According to preliminary tests conducted under physiological conditions, the GdL2 nanoparticles have some potential as contrast agent provided their stability can be increased.