The penetration of liquid metals into the grain boundaries of a solid metal can lead to severe embrittlement of the solid. The mechanism of wetting at the grain boundary, which is the precursor process of liquid metal embrittlement, is still far from being well understood. We report here our in-situ observations of the propagation process of liquid Ga into the grain boundaries of aluminium bicrystals and polycrystals. The experiments were performed using synchrotron radiation micro-radiography. This imaging technique is well adapted for the in-situ, non-destructive bulk characterisation of such a phenomenon due to its suitable spatial and temporal resolutions. Moreover, the large difference in absorption coefficients of Al and Ga is such that sub-micrometer thick liquid films are observable, allowing the analysis of the kinetics to be achieved. Therefore, the experiments show that the gallium penetration front in aluminium bicrystal propagates roughly in linear time dependence. On the contrary, the Ga penetration along the grain boundaries of polycrystals exhibits important discontinuities.