Manganese superoxide dismutase cycles between the Mn-III and Mn-II states to produce oxygen and hydrogen peroxide from superoxide. The formation of an adduct has been suggested, but its nature remains questionable because both [(MnOO-)-O-II] and [(MnOO2-)-O-III] redox states have been proposed. Study of the reactivity of superoxide with manganese complexes is of current interest. The reaction of [(L)Mn-II](2+) [L = N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine] with potassium superoxide has been investigated at low temperature in an anhydrous solvent using various techniques. Upon the addition of ca. 2 equiv of potassium superoxide, the [(L)Mn-II](2+) colorless solution turned blue and the UV-vis spectrum displayed a band at 590 nm (165 M-1 cm(-1)) and a shoulder at 430 nm (100 M-1 cm(-1)). Electrospray ionization mass spectrometry showed a peak (m/z = 434.1) assigned to [(L)MnO2](+). The X-band electron paramagnetic resonance spectrum parallel mode displayed a six-line signal separated by 6.6 mT and centered at 86 mT (g = 8.1). These results support the formation of an [(MnOO)-O-III](+) adduct.