The molecular motion of the planar bis(maleonitriledithiolato)nickel anion, Ni(mnt)(2)(-), has been studied as a function of temperature using electron spin resonance (ESR) in several polar solvents; they are ethyl alcohol, eugenol, dimethyl phthalate, tri-n-butyl phosphate, tris(2-ethyl-hexyl)phosphate, diglyme, and a dimethylformamide-chloroform mixed solvent. Calculated spectra in agreement with the experimental X-band spectra are obtained using axially symmetric reorientation when the long in-plane axis is the unique (parallel) axis of the rotational diffusion tensor with D-parallel to/D-perpendicular to = 3.0-4.0; D-parallel to and D-perpendicular to are the diffusion constants for reorientation about the parallel and perpendicular axes, respectively. The reorientational model required for the simulations is either in or close to the Brownian rotational diffusion limit. In the slow motional (low temperature) region, the spectra can be simulated using the glassy g values. As the temperature increases, however, agreement is obtained only if the intermediate g factor, g, for the non-axially symmetric Zeeman interaction increases while g,, g,, and the motional model remain unchanged; this scheme and others for which g, and g, are possibly temperature-dependent are discussed. The values of D-perpendicular to from the simulations are in general agreement with those from earlier analyses of the width of the central spectral feature. The simulations and width analyses indicate (as do electrochemical, conductivity, and vapor-phase osmometry data) that the paramagnetic species reorienting in solution has a shape similar to that of the Ni(mnt)(2)(-) ion.