The kinetics of hydrolysis of tripolyphosphate by purified exopolyphosphatase from Saccharomyces cerevisiae cytosol has been studied in the presence of Mg2+. Two kinetic models suggesting the formation of complexes of tripolyphosphate and the enzyme with Mg2+ are compared. Both models suggest that only enzyme--substrate complexes containing Mg2+ and tripolyphosphate simultaneously are able to hydrolyze the tripolyphosphate. The first model suggests that the enzyme is able to bind to Mg2+ independently from substrate binding. The second model does not consider this possibility, but suggests that both complexes containing tripolyphosphate and Mg2+ in proportion 1:1 and 1:2 can serve as the reaction substrates. The description of the experimental data by both models is essentially the same. The complex containing tripolyphosphate and Mg2+ in proportion 1:1 is optimal for the enzyme activity, the complex containing tripolyphosphate and Mg2+ in proportion 1:2 being hydrolyzed at a lower rate.