Thin compact La-Sr-Co-O composite films consisting of (La,Sr)CoO3-delta and (La,Sr)(2)CoO4+delta crystallites were prepared by compositional phase separation during pulsed laser deposition. While the total La: Sr ratio is kept constant at 70: 30, different phase ratios and crystallite sizes are investigated. The effect of hetero-interfaces on the oxygen reduction reaction (ORR) kinetics was investigated by impedance spectroscopy. The enhancement relative to the appropriate reference activity is quantified in terms of the triple phase boundary (TPB) length specific activity k(TPB)', resulting in k(TPB)' = 2-9 . 10(-13) cm(2)/s at 600 degrees C. A metastable Co-deficient perovskite phase La0.7Sr0.3Co0.9O3-delta formed in La-Sr-Co-O films under certain conditions was found to be highly active with an effective rate constant k(q) = 3 . 10(-6) cm/s at 600 degrees C, representing one possible source for the activity enhancement in these composite films. (C) The Author(s) 2017. Published by ECS. All rights reserved.