A O-17 NMR study (I = 1.0 M, NaClO4 or NaOSO2CF3, 25.0 degrees C) of solvent exchange in labeled p- and t-[Co(tren)(NH3)OH2](3+), [Co(tren)(OH2)(2)](3+), [Co(cyclen)(OH2)(2)](3+), and [Co(N-Mecyclen)(OH2)(2)](3+) ions (ca. 30% O-17) in aqueous solution has shown that loss of coordinated OH2 is slow for all of the complexes (k(ex)/s(-1) = 1.1 x 10(-5), 1.2 x 10(-5), 3.7 x 10(-5) (p-site)/8.7 x 10(-6) (t-site), 2 x 10(-4), and 2 x 10(-4), respectively). Values of k(ex) for solvent exchange in [Co(tren)(OH)(2)](+) have been determined as 9.7 x 10(-5) s(-1) (p-site) and 2.2 x 10(-7) s(-1) (t-site) Coordinated OH- in both p- and t-[Co(tren)(NH3)OH](2+) also exchanges only slowly with solvent (k(ex)/s(-1) = 1.7 x 10(-4), and <1 x 10(-6), respectively), whereas exchange of coordinated solvent in the aqua-hydroxo complexes [Co(tren)(t-OH2)(p-OH)](2+), [Co(cyclen)(OH2)OH](2+), and [Co(N-Mecyclen)(OH2)OH](2+) is much more rapid (k(ex)/s(-1) = 0.03 (p-site)/0.01 (t-site), 12 and 15, respectively). Ligand-OH- exchange in these latter systems is interpreted as occurring via (indirect) S(N)1(CB)-type processes on the corresponding aqua complexes : [Co-(amine)OH](2+) reversible arrow [Co(amine-H)OH2](2+) --> exchange. This type of pathway is seen to be more efficient when leaving-group departure is synchronous with proton transfer, and this appears to be more important for exchange in the cyclen and N-Mecyclen complexes where a reasonably acidic, adjacent syn NH proton is involved.