Layered-layered oxides of the type xLi2MnO3·(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 4.5V 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°C) confirmed the activation of Li2MnO3. The main signal at 615cm−1 shifted to higher wavenumbers upon charging. After reaching 4.4V vs. Li+/Li, this shift grew significantly, which is in good agreement with the onset of the potential plateau in both Li2MnO3 and xLi2MnO3·(1− x)LiMO2, and is assigned to the partial formation of a spinel-like phase.