The complex formation equilibria between pH 2 and 11.2 in Co(II)-L-O-2 ternary systems (L = histamine, glycylhistamine, and sarcosylhistamine) have been studied by pH-metric, spectrophotometric, and H-1-NMR spectroscopic methods. In contrast to earlier findings, we detected several protonation states of oxygen-carrying complexes in the pH range 7-11.2. The active species in oxygen uptake is the CoL2 complex already suggested in the Co(II)-histamine-O-2 system; however, in the case of pseudopeptides, both CoLH-1 and COL2H(-1) complexes can take up oxygen. The H-1-NMR study revealed stereoisomerism of oxygenated species in the same protonation states, undergoing slow ligand exchange. In the case of the Co-2(Hist)(4)(OH)(O-2) complex, among the 20 possible geometrical isomers, at least six can be distinguished by their imidazole proton signals. The most abundant isomers have axial-equatorial imidazole coordination, presumably on steric grounds. The stability constants determined by pH studies for the above systems proved that (4N)-coordinated species have a greater affinity for oxygen uptake than the (3N)-coordinated ones. They also showed the stabilizing effect of the deprotonated amide nitrogen in the oxygenated complexes of pseudopeptides.