This paper is an edition of an article by Władysław Kotwicz (1872–1944) entitled "Les voyelles longues dans les langues altaïques", which the author could not publish himself during wartime and did not live to publish after the War was over. The edition is designed to read almost as if published by Kotwicz, but without falsifying the actual manuscript. Also, a brief archival description is provided and the history of the last four years of the text has been reconstructed, based mostly on Kotwicz’s correspondence.