SrCo_0.8Fe_0.2O_3-δ is a controversial material whether it is used as an oxygen permeable membrane or as a cathode of solid oxide fuel cells. In this paper, carefully synthesized powders of perovskite-type Sr_xCo_0.8Fe_0.2O_3-δ (x = 0.80–1.20) oxides are utilized to investigate the effect of A-site nonstoichiometry on their electrochemical performance. The electrical conductivity, sintering property and stability in ambient air of Sr_xCo_0.8Fe_0.2O_3-δ are critically dependent on the A-site nonstoichiometry. Sr_1.00Co_0.8Fe_0.2O_3-δ has a single-phase cubic perovskite structure, but a cobalt-iron oxide impurity appears in A-site cation deficient samples and Sr₃(Co, Fe)₂O_7-δ appears when there is an A-site cation excess. It was found that the presence of the cobalt-iron oxide improves the electrochemical performance. However, Sr₃(Co, Fe)₂O_7-δ has a significant negative influence on the electrochemical activity for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The peak power densities with a single-layer Sr_1.00Co_0.8Fe_0.2O_3-δ cathode are 275, 475, 749 and 962 mW cm⁻² at 550, 600, 650 and 700 °C, respectively, values which are slightly lower than those for Sr_0.95Co_0.8Fe_0.2O_3-δ (e.g. 1025 mW cm⁻² at 700 °C) but much higher than those for Sr_1.05Co_0.8Fe_0.2O_3-δ (e.g. only 371 mW cm⁻² at 700 °C). This remarkable dependence of electrochemical performance of the Sr_xCo_0.8Fe_0.2O_3-δ cathode on the A-site nonstoichiometry reveals that lower values of electrochemical activity reported in the literature may be induced by an A-site cation excess. Therefore, to obtain a high performance of Sr_xCo_0.8Fe_0.2O_3-δ cathode for IT-SOFCs, an A-site cation excess must be avoided.