New octahedral rhenium cluster complexes [{Re(6)Q(8)} (SO3)(6)](10-) (Q= S or Se) were synthesized starting from [{Re(6)Q(8)}(H2O)(4)(OH)(2)]center dot 12H(2)O. The complexes were crystallized as sodium salts and characterized by X-ray single-crystal diffraction and elemental analyses, IR, UV/vis and luminescence spectroscopies. Magnetic relaxation data demonstrate the complex formation of the cluster units with gadolinium ions. The analysis of the magnetic relaxation rates measured at various Gd:cluster ratios and different concentrations revealed the conversion of the aggregates (Gd-x[{Re6Se8}(SO3)(6)])(y))(n-) into a nanoparticulate form even at x = 1 and y >= 1. Thus, the self-assembly of the cluster units into the nanoparticles is greatly facilitated by counterion binding with sodium cations. The concentration conditions were optimized for the formation and hydrophilization of NaxGdy[{Re(6)Q(8)}(SO3)(6)]-based colloids with the magnetic relaxivity values of r(1(2)) = 21.0(24.1) and r(1(2)) = 25.9(29.8) mM(-1) s(-1) for the {Re6S8}(2+) and {Re6Se8}(2+) derivatives, respectively.