Variable-temperature (at 4.7 and 9.4 T) and variable-pressure (9.4 T) O-17 nuclear relaxation rates were measured for the Eu(TI) aqua ion. Variable-temperature H-1 nuclear magnetic relaxation dispersion(NMRD) profiles were recorded. In addition, EPR spectra of the Eu(II) aqua ion are reported as a function of temperature at 0.34, 2.7, 5.4, and 8.1 T. The simultaneous fit of the nuclear and electronic relaxation rate data results in k(ex)(298) = 4.4 x 10(9) s(-1) for the water exchange rate with activation parameters Delta H-double dagger = 15.7 kJ mol(-1) and Delta S-double dagger = -7.0 J K-1 mol(-1), Delta V-double dagger = -11.3 cm(3) mol(-1) for the volume of activation for exchange, and tau(R)(298) = 16.3 ps for the rotational correlation time. Water exchange at Eu-(aq)(2+) occurs via an associative mechanism and has the highest rate ever measured at an aqua ion by magnetic resonance.:The high-field EPR spectra, reported here for the first time for divalent Eu, show hyperfine coupling to Eu-151 and Eu-153 and the coupling constants are determined (37.3 and 16.4 G for Eu-151 and Eu-153, respectively). The electronic relaxation times, T-1e and T-2e, are longer than for the isoelectronic Gd(III) aqua ion. The implications of these results for ligand exchange at Ca(II) and for magnetic resonance imaging contrast agents are discussed.