SrCo(0.8)Fe(0.2)O(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(x)Co(0.8)Fe(0.2)O(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(x)Co(0.8)Fe(0.2)O(3-δ) are critically dependent on the A-site nonstoichiometry. Sr(1.00)Co(0.8)Fe(0.2)O(3-δ) has a single-phase cubic perovskite structure, but a cobalt-iron oxide impurity appears in A-site cation deficient samples and Sr(3)(Co, Fe)(2)O(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, Sr(3)(Co, Fe)(2)O(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(1.00)Co(0.8)Fe(0.2)O(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(0.95)Co(0.8)Fe(0.2)O(3-δ) (e.g. 1025 mW cm(-2) at 700 °C) but much higher than those for Sr(1.05)Co(0.8)Fe(0.2)O(3-δ) (e.g. only 371 mW cm(-2) at 700 °C). This remarkable dependence of electrochemical performance of the Sr(x)Co(0.8)Fe(0.2)O(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(x)Co(0.8)Fe(0.2)O(3-δ) cathode for IT-SOFCs, an A-site cation excess must be avoided.