 When the Norwegian Campaign ended and the German occupation of Norway was a reality, Tronstad became involved in resistance work, largely based around the Norwegian Institute of Technology. He was associated with the radio agent group Skylark B, which had regular contact with London from January 1941. Among other things, he sent reports on the interest shown by German authorities in the heavy water plant at Vemork. One source of this information was Tronstad's old companion, Jomar Brun, still in charge of the plant, who also helped people to flee the country by sea, via the Møre coast. After exposure of the group in September 1941, Tronstad himself had to flee the country. Another resistance member, who had already been jailed, managed to warn Tronstad, who travelled from Trondheim to Oslo by train. The following day, the Gestapo visited his house to arrest him. After a few days in hiding, Tronstad was driven by car to Østfold, and then travelled on foot to Töcksfors via Ørje. From there he continued by plane to England, rejecting an offer of a civilian job in Sweden.
