Electron spin echo envelope modulation studies aimed at characterizing the superhyperfine coupling between protons of axially bound water molecules and the nickel ion in NiIII(CN)4(H2O)2 - were carried out. X-band, two-pulse ESEEM experiments showed that this coupling is characterized by a large anisotropic component resulting in 0.1-1.6-MHz shifts of the vα + vβ proton sum combination peak from twice the Larmor frequency. A means of reducing these data into a field profile constructed by plotting these frequency shifts as a function of magnetic field strength across the EPR absorption envelope is presented. Computer simulations of typical field profiles are shown to be sensitive to the effective Ni-H dipole-dipole distance, the orientation of the principal axis of the hyperfine coupling tensor with respect to the g-tensor, and to a lesser extent, the Fermi contact or scalar hyperfine coupling term. For water coordinated to NiII(CN)4(H2O)2 -, analysis of the field profile yields an effective dipole-dipole distance of 2.4 Å, an angle describing the orientation of A∥ for the proton superhyperfine coupling tensor with respect to the g3 axis of 12°, and a scalar coupling ≤4 MHz. These results are discussed in the context of recent ENDOR studies of the nickel site in Ni-Fe hydrogenases.