An Electron Paramagnetic Resonance (EPR) study of glasses and magnetically dilute powders of [Gd(DTPA)(H2O)]2−, [Gd(DOTA)(H2O)]-, and macromolecular Gd-P792 complexes was carried out at X-, Q-band and 240 GHz DTPA=diethylenetriaminepentaacetato; DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetato). The results show that the zero-field splitting parameters for these complexes are quite different in a powder as compared to the frozen aqueous solution. In several complexes an inversion of the sign of the axial component D of the zero field splitting is observed, indicating a significant structural change. In contrary to what has been expected, powder sample do not allow a more precise determination of the static ZFS parameters in the powders obtained by liophilization. The results obtained in glasses are more relevant to the problem of electron spin relaxation in aqueous solution than those obtained from powders.