Koserius graduated from the
Swedish Air Force Flying School in 1962 and was commissioned as an officer with the rank of second lieutenant. He was promoted to lieutenant in 1964. On 21 June 1978, Koserius graduated from
Flygvapnets Krigsskola (F 20) in
Uppsala, and was awarded commanding officer of F 20's silver plaque and the Swedish Association of Army, Navy and Air Force Officers's (
Svenska officersförbundet) book prize for good study achievements. He completed the Higher Course at the
Swedish Armed Forces Staff College from 1981 to 1983 and was promoted to major in 1983. Koserius served in the
Defence Staff and in the
Air Staff from 1983 to 1985. He was head of Section 1 at
Uppland Wing from 1986 to 1988, and was promoted to lieutenant colonel in 1987 and served in the Air Staff from 1988. On 1 April 1992, Koserius was appointed permanent head of the same unit in the Air Staff and on 1 July the same year, he was promoted to senior colonel. He was succeeded by colonel Nils Ullgren on 1 January 1993 when Koserius was posted to staff of the
Middle Military District (Milo M) as chief of the Operations Command. On 1 October 1995, Koserius took command of the
Central Air Command, a position he held until 1996. Koserius served in these positions until 30 June 2001. From 27 June 2001 to 11 June 2002, Koserius served as head of the Swedish Delegation to the
Neutral Nations Supervisory Commission (NNSC). ==Death==