The reaction catalyzed by IMP dehydrogenase (IMP: NAD+ oxidoreductase EC 1.2.1.14) from Aerobacter aerogenes has been investigated kinetically at pH 8.1 as a three reactant system by means of steady-state velocity studies in the absence of products, as well as by inhibition studies using products and substrate analogues. The mechanism appears to be a partially random one in which IMP and K+ can bind randomly to the free enzyme while NAD does not react unless K+ or both K+ and IMP are present on the enzyme. While the steady-state velocity data can be analysed adequately on the basis that rapid equilibrium conditions apply, this is only an approximate description of the mechanism since product inhibition studies indicate that there is a significant concentration of an enzyme-XMP (enzyme-K-XMP) complex in the steady-state.