Gd(III) (S = 7/2) polyaminocarboxylates, used as contrast agents for Magnetic Resonance Imaging (MRI), were studied in frozen solutions by High-Frequency-High-Field Electron Paramagnetic Resonance (HF-EPR). EPR spectra recorded at 240 GHz and temperatures below 150 K allowed the direct and straightforward determination of parameters governing the strength of zero-field splitting (ZFS). For the first time, a correlation has been established between the sign of the axial ZFS parameter, D, and the nature of the chelating ligand in Gd(III) complexes: positive and negative signs have been observed for acyclic and macrocyclic complexes, respectively. Furthermore, it has been shown that complexes of the less symmetric acyclic DTPA derivatives possess a substantial rhombicity, E, in contrast to the more symmetric macrocyclic DOTA derivatives, where E is negligible. The results obtained are compatible with recent results of liquid-state EPR and allowed to simulate H-1 Nuclear Magnetic Relaxation Dispersion (NMRD) profiles with more directly physically meaningful EPR and NMR parameters over the full frequency range from 0.01 to 50 MHz.