NO2 adsorption at a BaO(100) surface is investigated by means of spin polarized GGA density functional theory. A periodic supercell procedure is employed, and two redox reaction channels are mapped out, involving two chemisorbed NO2 molecules per supercell. The chemisorption is studied in two subsequent steps. The reaction paths are initiated by NO2 adsorption in the form of a nitrite over a Ba2+ site. This generates an electron hole among the surrounding surface oxygen atoms. A reaction path branching occurs as the second NO2 either (a) acts as surface oxidant, forming a surface nitrite-peroxide pair by releasing NO(g), or (b) binds to an O-surf(-) Site to form a formal surface nitrate. A redox reaction involving surface nitrite-nitrate interconversion is also addressed. The computed results are employed to interpret experimental observations of surface nitrites, peroxides, NO(g) desorption, and surface Ba(NO3)(2) formation. The understandings are discussed in the context of the NOx storage concept of lean-burn catalysis.