Lanthanum manganite, the air electrode material of solid oxide fuel cells, is well known for its complicated nonstoichiometry, which depends upon oxygen partial pressure and temperature. This nonstoichiometry has a relationship with electrode reactivity or the exchange current density because it affects the oxygen activation step. The electrode reactivity is usually studied in terms of its exchange current density, in other words, under a nonpolarized (equilibrium) state. This paper aims to elucidate the effect of oxygen nonstoichiometry on cathode reactivity under polarized states. The complex impedance was measured on an Ar-O-2/porous La0.9Sr0.1MnO3+delta/yttria-stabilized zirconia electrode under steady-state polarization in the oxygen partial pressure range of 10(2)-10(5) Pa and a temperature range of 873-1273 K. The results suggest that oxygen nonstoichiometry affects the cathode reactivity both under polarized and nonpolarized states through the exchange current density.