Phase instabilities of the RBaCo2O5+delta (R = Y and Ho) layered-perovskites and their decompositions into RCoO3 and BaCoO3-z at 800 degrees in air were investigated. The phase instability will restrict their high temperature applications such as cathodes in solid oxide fuel cells (SOFC). However, appropriate amount of Sr substitution (>= 60% for R = Y and >= 70% for R = Ho) for Ba successfully stabilized the R(Ba1-xSrx)Co2O5+delta phase at elevated temperatures. This can be explained to be due to the decrease in oxygen vacancies in the R-O layer, decrease in R-O bond length, and consequent improvement in structural integrity. In addition, the Sr substitution (x = 0.6-1.0) for Ba provided added benefit with respect to the chemical stability against Ce0.8Gd0.2O1.9 (GDC) electrolyte, which is a critical requirement for the cathodes in SOFC. Among the various compositions investigated, the Y(Ba0.3Sr0.7)Co2O5+delta + GDC composite cathode delivered the optimum electrochemical performances with a stable phase, demonstrating the potential as a cathode in SOFC. (C) 2013 Elsevier B.V. All rights reserved.