Complementary in operando X-ray radiography and neutron radiography measurements were conducted to investigate and visualize the initial lithiation in silicon-electrode lithium-ion batteries. By means of X-ray radiography, a significant volume expansion of Si particles and the Si electrode during the first discharge was observed. In addition, many Si particles were found that never undergo electrochemical reactions. These findings were confirmed by neutron radiography, which, for the first time, showed the process of Li alloying with the Si electrode during initial lithiation. These results demonstrate that complementary X-ray and neutron radiography is a powerful tool to investigate the lithiation mechanisms inside Si-electrode based lithium-ion batteries.