Mössbauer Spectroscopy (MS) is really suitable to study local electronic structures. Its hyperfine parameters, isomer shift (IS) and quadrupole splitting (QS) allow to characterize the oxidation state and coordination of the probed element. So, the capabilities of this powerful technique have been highlighted for the study of electron transfer occurring during electrochemical or catalytic processes. Several examples illustrate how MS can be used for the determination of reaction mechanisms in new electrode materials of Li-ion batteries (Sb or Sn-based, Ti oxides) or reforming catalysts (supported bimetallic PtSn catalysts). Deeper insight into the mechanisms determining the electrochemical or catalytic performances can be expected.