Latent Thermal Energy Storage (LTES) systems are based on Phase Change Materials (PCMs), and they represent an innovative possibility to store thermal energy. PCMs are characterised by a nearly isothermal solid-liquid phase-change and they can store a larger amount of thermal energy if compared to sensible systems for temperatures close to the phase transition. These advantages are very attractive: many researchers are working on this topic to fight against the climate change and global warming by increasing the share of renewable energy, the efficiency of energy systems and reducing energy losses. For these reasons, it is important to properly design LTESs which necessitates to assess the behaviour of PCMs. Along with PCM temperature, the liquid fraction (share of PCM in a liquid state compared to the total volume) is maybe the most important parameter, which can help in characterising LTESs. A precise quantitative methodology to measure the liquid evolution of the PCM is still missing. Here a new methodology to fill this gap is proposed relying on non-destructive X-ray Computed Tomography (XCT) technology. A laboratory based XCT is used in the present work to record images of the solidification process of eicosane: an organic PCM, which melts at around 37°C. After image analyses, liquid fraction evolution over time is extracted including an associated experimental error estimation. The possibility, through X-rays to see inside the sample opens a lot of opportunities in this field of research.