Investigations on the mechanism of the IMP dehydrogenase (IMP: NAD+ oxidoreductase, EC 1.2.1.14) reactions have been made at pH 7.0 by measuring rates of isotope exchange at chemical equilibrium with K+ maintained at a constant concentration. The results are generally in accord with the conclusions reached on the basis of the steady-state kinetic data obtained previously and confirm that there is random addition of IMP and NAD to the enzyme. The data also indicate clearly that at pH 7.0 catalysis is faster than the rate of IMP and/or XMP release which is rate limiting for the reaction sequence. The binding of IMP to the enzyme at pH 8.1 has been demonstrated to occur in the absence of both K+ and NAD and id independent of the K+ concentration.