The ion-nuclear distance of Gd(III) to a coordinated water proton, r(Gd-H), is central to the understanding of the efficacy of gadolinium-based MRI contrast agents. The dipolar relaxation mechanism operative for contrast agents has a 1/r(6) dependence. Estimates in the literature for this distance span 0.8 Angstrom (2.5-3.3 Angstrom). This study describes a direct determination of rGd-H using the anisotropic hyperfine constant T-perpendicular to determined from pulsed ENDOR spectra. Five Gd(III) complexes were examined: [Gd(H2O)(8)](3+), [Gd(DTPA)(H2O)](2-), [Gd(BOPTA)(H2O)](2-), MS-325, and [Gd(HP-DO3A)(H2O)]. The distance, r(Gd-H), was the same within error for all five complexes: 3.1 +/- 0.1 Angstrom. These distance estimates should aid in the design of new contrast agents, and in the interpretation of other molecular factors influencing relaxivity.