Water exchange kinetics of [Ln(L)(H2O)(2)](x) complexes (Ln = Pr, Nd, Dy, Tm, and Yb; L = DO3A and DTTA-Me) were studied by O-17 NMR spectroscopy as a function of temperature, pressure, and frequency and by H-1 nuclear magnetic relaxation dispersion. Water exchange rate constants of both complexes show a maximum at dysprosium. Water exchange on negatively charged complexes of the acyclic DTTA-Me ligand is much faster than on the neutral complexes of the macrocyclic DO3A. Small activation volumes |Delta double dagger| < 1 cm(3) mol-1 measured for water exchange on [Ln(DO3A)(H2O)(2)] indicate an interchange type of mechanism (I) for the lanthanide complexes studied. In the case of [Ln(DTTA-Me)(H2O)(2)]-, a change in mechanism is detected from a dissociative mechanism (D, Delta double dagger = 7 cm(3) mol(-1)) for complexes with larger ions (Pr to Gd) to an interchange mechanism (I-d, I; Delta double dagger = +1.8 and +0.4 cm(3) mol(-1)) for complexes with smaller ions (Dy and Tm).