After the outbreak of World War II, during the German occupation of Poland, Hiż continued his education in the underground university until 1944. At the time, he was appointed an assistant to Kotarbiński and Łukasiewicz, and ran the classes in their absence. In the wartime he made friends with Jerzy Pelc, Klemens Szaniawski, Jan Strzelecki, Andrzej Grzegorczyk and Krzysztof Kamil Baczyński.