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 (1)H-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 CoL(2) complex already suggested in the Co(II)-histamine-O(2) system; however, in the case of pseudopeptides, both CoLH(-)(1) and CoL(2)H(-)(1) complexes can take up oxygen. The (1)H-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.