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 A (2.5-3.3 A). This study describes a direct determination of r(Gd)(-)(H) using the anisotropic hyperfine constant T( perpendicular ) determined from pulsed ENDOR spectra. Five Gd(III) complexes were examined: [Gd(H(2)O)(8)](3+), [Gd(DTPA)(H(2)O)](2)(-), [Gd(BOPTA)(H(2)O)](2)(-), MS-325, and [Gd(HP-DO3A)(H(2)O)]. The distance, r(Gd)(-)(H), was the same within error for all five complexes: 3.1 +/- 0.1 A. These distance estimates should aid in the design of new contrast agents, and in the interpretation of other molecular factors influencing relaxivity.