In the objective of optimizing water exchange rate on stable, nine-coordinate, monohydrated Gd^III poly(amino carboxylate) complexes, we have prepared monopropionate derivatives of DOTA⁴⁻ (DO3A-Nprop⁴⁻) and DTPA⁵⁻ (DTTA-Nprop⁵⁻). A novel ligand, EPTPA-BAA³⁻, the bisamylamide derivative of ethylenepropylenetriamine-pentaacetate (EPTPA⁵⁻) was also synthesized. A variable temperature ¹⁷O NMR study has been performed on their Gd^III complexes, which, for [Gd(DTTA-Nprop)(H₂O)]²⁻ and [Gd(EPTPA-BAA)(H₂O)] has been combined with multiple field EPR and NMRD measurements. The water exchange rates, k_ex²⁹⁸, are 8.0 × 10⁷ s⁻¹, 6.1 × 10⁷ s⁻¹ and 5.7 × 10⁷ s⁻¹ for [Gd(DTTA-Nprop)(H₂O)]²⁻, [Gd(DO3A-Nprop)(H₂O)]⁻ and [Gd(EPTPA-BAA)(H₂O)], respectively, all in the narrow optimal range to attain maximum proton relaxivities, provided the other parameters (electronic relaxation and rotation) are also optimized. The substitution of an acetate with a propionate arm in DTPA⁵⁻ or DOTA⁴⁻ induces increased steric compression around the water binding site and thus leads to an accelerated water exchange on the Gd^III complex. The k_ex values on the propionate complexes are, however, lower than those obtained for [Gd(EPTPA)(H₂O)]²⁻ and [Gd(TRITA)(H₂O)]⁻ which contain one additional CH₂ unit in the amine backbone as compared to the parent [Gd(DTPA)(H₂O)]²⁻ and [Gd(DOTA)(H₂O)]⁻. In addition to their optimal water exchange rate, [Gd(DTTA-Nprop)(H₂O)]²⁻ has, and [Gd(DO3A-Nprop)(H₂O)]⁻ is expected to have sufficient thermodynamic stability. These properties together make them prime candidates for the development of high relaxivity, macromolecular MRI contrast agents.