The enhancement of electroactivity and stability of solid oxide fuel cell (SOFC) cathodes is an open issue for the scientific community. Only a careful experimental approach, coupled with advanced modelling and interpretation, allows for a comprehensive understanding and optimization of materials not completely exploited, such as strontium-doped lanthanum manganite (LSM). This paper, as first part of a couple of articles, presents experimental measurements of porous LSM cathodes and gives an interpretation of the oxygen reduction reaction (ORR) mechanism. The experimental section takes into account the effect of cell geometry and different electrode microstructures resulting from sintering the cathodes at different temperatures. Electrodes are tested with electrochemical impedance Spectroscopy (EIS) in three-electrode configuration and the influence of cathodic dc bias, oxygen partial pressure and temperature is analysed. The effects of dc overpotential on the processes are remarkable and a particular attention is dedicated to this parameter. The analysis of experimental data with equivalent circuits identifies a critical overpotential between 0.15 and 0.2 V. Around this value the system undergoes a modification in the ORR mechanism and a new reaction path is identified. Analogous conclusions are achieved in the second paper by using a detailed mechanistic modelling approach. (C) 2017 Elsevier B.V. All rights reserved.