T-1 relaxation times of water in aqueous solutions of three types of paramagnetic transition metal-substituted heteropoly complexes were measured at the magnetic field of 10.7 MHz. The three types of complexes had the following chemical formulas : I, alpha-[XW(11)O(39)Z(H2O)](n-), where X = P or Si and Z = Fe3+ or Mn2+; II, alpha(2)-[P(2)W(17)O(61)Z(H2O)](m-), where Z = Mn2+ or Fe3+; III, [GdH2W10O36](7-). In the first two groups of compounds, paramagnetic transition metals exist in identical ligand environments, where each is surrounded by five oxide ions and one water molecule. The individual heteropoly complexes, however, differ in net charge, size, and metal, allowing the total relaxivity to be separated into individual components. Therefore, effects of the size, the net complex charge, the metal type, and the rotational correlation time on the relaxivity were determined independently. The results are compared to the existing theories of relaxivity in solution.