The kinetics of oxygen exchange between TcO4- and solvent water has been examined at 25 degrees C using O-17 NMR measurements. The data suggest the rate equation k(obs) = k(H)[H+](2) + k(o). The rate constant for the [TcO4-][H+](2) term is 32.6 M-2 s(-1), which is a factor of 2 smaller than that for ReO4-. Oxygen exchange with solvent water at neutral pH is extremely slow for both TcO4- and ReO4- with the acid independent rate constants being 5 x 10(-9) M-1 s(-1) and 1.4 x 10(-8) M-1 s(-1), respectively. In the presence of the complexing citrate ion the ReO4--H2O exchange is dramatically increased in the acid independent region, but this ligand has little effect on TcO4-. This suggests that ReO4-, but not TcO4-, is capable of expanding its coordination number. In agreement with this concept of coordination number expansion, there is no significant [OH-] dependent term in the rate law for TcO4-, as there is for ReO4-.