Under fully reduced conditions, reassociation kinetics of CO were studied in several terminal oxidases containing copper in their binuclear center. The purified Paracoccus denitrificans ba3-type quinol oxidase was found to recombine with CO monophasically (tau 25-30 ms) like oxidases of the bo type from Escherichia coli, the caa3 type from Bacillus halodurans FTU, and the bo type from Methylobacillus flagellatum KT. Oxidase of the aa3 type from bovine heart recombined with CO monophasically at a higher rate (tau 16-19 ms) than the studied copper-containing bacterial oxidases. After prolonged incubation in the presence of CO, oxidases of the ba3 and aa3 types changed their CO-binding properties. The contribution of the slow component was diminished while new fast components arose. Measurement of the metal content in the oxidases indicated that during the incubation, the enzymes lost their copper, the process being accompanied by the appearance of a fast CO recombination rate resembling that of the non-copper oxidases of the bd type from E. coli and the bb type from Bacillus halodurans FTU. This points to a role of copper in CO binding by terminal oxidases.