The phenomenon of grain boundary penetration (GBP) of liquid Ga along grain boundaries (GB) of Al bicrystals is investigated by synchrotron radiation X-ray microradiography. From the three different types of bicrystals studied, only the one with the highest GB energy showed GBP in the absence of applied external stress. In situ observations of the penetration process reveal a linear propagation of the penetration front, accompanied by a continuous thickening of the wedge-shaped Ga layer in the GB. The experimental results demonstrate that GBP kinetics are strongly influenced by very weak levels of stress and tend to indicate that such stresses may be a prerequisite for the formation of nanometric penetration layers.