The behaviour of Co(NO3)2 and 1,4,7,10,13,16,19,22,25,28-decaazacyclotriacontane aqueous solutions towards molecular dioxygen has been examined over a large range of ratios R between the ligand and cobalt(II) concentrations and in the presence of different types of anions at high concentrations. Both binuclear, Co2L(O2)(OH), and mononuclear, Co(HmL)(O2), peroxo complexes are formed depending on the value of R. The formation of the binuclear species is rapid, while that of the mononuclear complexes is slow. Phosphate anions (Y) enter into the bi- and mono-nuclear peroxo complexes, giving CoL(O2)Y and CoL(O2)Y species. The kinetics of dioxygen uptake under various experimental conditions has been studied and reaction schemes are proposed.