Azurin is a small blue copper protein in the electron transfer chain of denitrifying bacteria. It forms a photolabile complex with nitric oxide (NO) at low temperatures. We studied the temperature dependence of the ligand binding equilibrium and the kinetics of the association reaction after photodissociation over a wide range of temperature (80-280 K) and time (10(-6)-10(2) s). The nonexponential rebinding below 200 K is independent of the NO concentration and is interpreted as internal recombination. The rebinding can be modeled with the Arrhenius law by using a single preexponential factor of 6.3 x 10(8) s-1 and a Gaussian distribution of enthalpy barriers centered at 23 kJ/mol with a width of 11 kJ/mol. Above 200 K, a slower, exponential rebinding process appears. The dependence of the kinetics on the NO concentration characterizes this reaction as bimolecular rebinding. The binding kinetics of NO to azurin show impressive analogies to the binding of carbon monoxide to myoglobin. We conclude that conformational substates occur not only in heme proteins but also in proteins with different active sites and secondary structures.