One molecule of the enzymatic intermediate histidinal is firmly bound per subunit of histidinol dehydrogenase (EC 188.8.131.52) and protected against decomposition. The dissociation rate constant of the histidinal–histidinol dehydrogenase complex is estimated as 2.5 × 10–5 S–1. Steady-state kinetic measurements studying the oxidation of histidinol to histidine and the reduction of histidinal to histidinol allow to calculate the association rate constants for histidinal. For both reactions the association rate constant is found as 1.9 × 106 M–1 S–1. Thus the dissociation constant of the histidinal–histidinol dehydrogenase complex is estimated to be of the order of 1.4 × 10–11 M.