In Pseudomonas carboxydovorans, CO dehydrogenase and hydrogenase were found in association with the cytoplasmic membrane in a weakly bound and a tightly bound pool. The pools could be experimentally distinguished on the basis of resistance to removal by washes in low-ionic-strength buffer. The tightly bound pool of the enzymes could be differentially solubilized under conditions leaving the electron transport system intact and with the nondenaturing zwitterionic detergent 3-(3-cholamidopropyl) dimethylammonio 1-propane-sulfonic acid (CHAPS) and the nonionic detergent dodecyl beta-D-maltoside. In vitro reconstitution of depleted membranes with the corresponding supernatants containing CO dehydrogenase led to binding of the enzyme and to reactivation of respiratory activities with CO. The reconstitution reaction required cations with effectiveness which increased with increasing ionic charge: monovalent (Li+), divalent (Mg2+, Mn2+), or trivalent (Cr3+, La3+). Reconstitution of depleted membranes with CO dehydrogenase was specific for CO-grown bacteria. Cytoplasmic membranes from H2- or heterotrophically grown Pseudomonas carboxydovorans had no affinity for CO dehydrogenase at all, indicating the absence of the physiological electron acceptor of the enzyme, which presumably is cytochrome b561, or another membrane anchor.