Superparamagnetic iron oxide particles in commercial products Resovist and SH U555C, used as magnetic resonance imaging contrast agents, are polydispersed and covered with a thick organic coating so that the average particle sizes were reported between 3 and 62 nm with different definitions. It is unclear which size determines the transverse relaxation rate 1/ T2 of water suspensions of such particles. Comparing the measured 1/ T2 and average sizes of different definitions determined from magnetization curve, transmission electron microscopy image, x-ray diffraction, and dynamic light scattering, it is found that the 1/ T2 behavior is basically determined by the diameter of bare single-crystal magnetic particles having the particle-volume-weighted average volume (about 14 or 9 nm for Resovist or SH U555C) and is slightly influenced by their thick organic coating. This is explained by the low partial density of the coating substance and the overwhelming water occupation adjacent to magnetic particles. © 2009 American Institute of Physics.