The process of grain boundary penetration of liquid Ga in Al polycrystals is characterized by synchrotron radiation X-ray microradiography. This experimental technique allows for non-destructive in situ characterization of dynamic processes in the bulk of millimetric size Al samples. Ga wetting layers as thin as 5–20 nm can be detected inside grain boundaries (GBs) which are favorably inclined with respect to the X-ray beam.The penetration process can be divided into three characteristic steps: (1) propagation and simultaneous thickening of the liquid film, (2) thickening discontinuities with fast propagation rates (higher than 5 mm/s) and (3) saturation of the layer thickness. Discontinuities can be attributed to the formation of intergranular cracks and result in rapid transfer and redistribution of the Ga along the GB network. Experimental results are strongly influenced by stress, even at very low stress levels.