The reduction of [99Tc]pertechnetate was studied as a function of pH in complexing media of pyrophosphate, methylene diphosphonate (MDP), and ethane-1, hydroxy-1 and 1-diphosphonate (HEDP). Tast (sampled d-c) and normal-pulse polarography were used to study the reduction of pertechnetate, and normal-pulse polarography (sweeping in the anodic direction) to study the reoxidation of the products. Below pH 6,TcO4- was reduced to Tc(III), which could be reoxidized to Tc(IV). Above pH 10, TcO4- was reduced in 2 steps to Tc(V) and Tc(IV), each of which could be reoxidized to TcO4-. Between pH 6 and 10 the results differed according to the ligand present. In pyrophosphate and MDP, TcO4- was reduced in 2 steps to Tc(IV) and Tc(III); Tc(III) could be reoxidized in 2 steps to Tc(IV) and TcO4-. In HDP, on the other hand, TcO4- was reduced in 2 steps to Tc(V) and Tc(III), and could be reoxidized to Tc(IV) and TcO4-. Additional waves were observed; they apparently led to unstable products. [The practical importance of the diphosphonate and pyrophosphate complexes of technetium lies in their medical use for radionuclide bone scanning.].