Lanthanum manganite, an air electrode material of solid oxide fuel cells, is well known for its complicated nonstoichiometry, which depends upon oxygen partial pressure. This nonstoichiometry is considered to have a relation to electrode activity as it affects oxygen activation and/or oxide ion sink sites, etc. However, its relation to cathode activity has nor yet been elucidated, mainly because of the lack of detailed data on oxygen nonstoichiometry. In this paper, we report the relation between oxygen nonstoichiometry and cathode activity based on measurements of the oxygen nonstoichiometry and complex-impedance of the system Ar, O-2/porous La(1-x)A(x)MnO(3+/-delta) (A = Sr, Ca)/YSZ, with p(O2) in the range of 1-10(5) Pa and temperature at 973-1273 K. When the nonstoichiometric oxygen content, 3+/-delta, increases, the electrode activity decreases and vice versa. These results suggest that vacant sites of cations and/or O are related to the reaction path of cathode reaction. (C) 2000 The Electrochemical Society. S0013-4651(00)05-046-1. All rights reserved.