The kinetics of CO reassociation with isolated BuciZZus FTU o-type oxidase and with solubilized membranes of Escherichia coli (GO102 strain) containing the d-type oxidase only, upon laser flash photolysis under reducing conditions, were studied. In both cases, kinetics are shown to be composed of three phases (r 35-70~s 0.25-0.5 ms and 2-5 ms). The spectra of the flash-induced absorbance changes of the first kinetic components proved to be characteristic of CO-o-and CO-b,, d-cytochrome complexes in Bat. FTU and E. co/i, respectively. The spectra of the second and the third components appeared to be nearly the same in But. FTU and E. coli with peaks for the former at 436437 and 590 nm and troughs at 419-420 and 569 nm; and for the latter with peaks at 436437 and 558-560 nm and troughs at 419420 and 575-578 nm. The similarity between the putative Na'-pumping Bat. FTU o-and E. coli d-type oxidases and their difference from the H'-motive Bat. FTU cuu3-and E. coli o-type oxidases are discussed.