The history of communist crimes in the USSR has been well elucidated. Nonetheless, a still under-investigated group of archival materials are files of the Soviet counterintelligence. One of its tasks was the surveillance of the foreign diplomats and consular representatives operating on the territory of the USRR. Even after the fall of the USSR and the opening of the archives, access to the materials of the communist special services was and is very difficult. The situation changed not very long ago. Open access to materials of the former GPU/NKVD/KGB was possible in Ukraine. In the Branch State Archive of the Security Service of Ukraine in Kyiv is a file continuing materials from the surveillance by the Soviet counterintelligence of the Polish diplomat Jan Karszo-Siedlewski, who was among others the head of the Polish consulates general in Kharkiv and Kyiv in 1932–1937. In this way, material that had been entirely inaccessible for researchers will be discussed in the present article.