Layered-layered oxides of the type xLi(2)MnO(3)center dot(1-x)LiMO2 (M=Mn, Ni, Co) have been postulated to contain Li2MnO3 domains which, upon electrochemical activation, give rise to a characteristic potential plateau at 4.5 V vs. Li+/Li. To improve our understanding of the complex reaction mechanisms at play, we applied in situ Raman microscopy to investigate the constituent Li2MnO3. Li2MnO3 synthesised via a two-step solid-state reaction was characterised by scanning electron microscopy and X-ray diffraction. Preliminary electrochemical tests and ex situ Raman microscopy showed the need for elevated temperatures to achieve activation. For the first time, in situ Raman microscopy (at 50 degrees C) confirmed the activation of Li2MnO3. The main signal at 615 cm(-1) shifted to higher wavenumbers upon charging. After reaching 4.4 V vs. Li+/Li, this shift grew significantly, which is in good agreement with the onset of the potential plateau in both Li2MnO3 and xLi(2)MnO(3)center dot(1-x)LiMO2, and is assigned to the partial formation of a spinel-like phase. (C) 2013 Elsevier Ltd. All rights reserved.