The modelling of electrode kinetics of solid oxide fuel cells is challenging, as the electrodes can change their composition and microstructure during operation at high temperature. Here we present results from in situ studies, applying time of flight secondary ion mass spectrometry (ToF-SIMS) to investigate compositional surface changes of lanthanum strontium manganate (LSM) model electrodes. Geometrically well-defined LSM electrodes with the composition La0.74Sr0.17Mn1.01O2.9 were deposited as thin films on yttria stabilised zirconia (YSZ) single crystals by pulsed laser deposition. As counter electrode, a porous platinum electrode was applied on the backside of the solid electrolyte. The electrochemical polarisation experiments were carried out inside the ToF-SIMS analysis chamber at 430. degrees C, and the ToF-SIMS measurements were performed-in contrast to former work-for the first time during electrical polarisation at elevated temperatures. By applying either a cathodic or anodic potential to the LSM cathode, enrichment or depletion of the different metallic constituents on both, the LSM and free YSZ surface, and within the LSM near surface region, were observed. After polarisation, the system relaxes towards the thermodynamic equilibrium state. Already after the first heat treatment, diffusion of manganese and strontium from the LSM electrode onto the free YSZ surface is observed. The concomitant spreading of the three-phase boundary seems to be one reason for electrode activation after polarisation. (C) 2017 Elsevier B.V. All rights reserved.