In the spring of 1940, Unteroffizier Ademeit was transferred to 3./JG 54 and participated in the Battle of Britain. He claimed his first victory on 18 September 1940 shortly afterwards he was shot down over the Channel. He bailed out and was rescued unharmed.

In June 1941, after the attack on the Soviet Union, he accompanied I./JG 54 to the Eastern Front. In quick succession he achieved aerial victories, promotions and awards. In the beginning of August 1944, Ademeit was appointed acting Kommodore of JG 54.

On 7 August 1944, Ademeit, flying a Focke Wulf Fw 190 A-5 (Werksnummer 5960 — factory number) pursued a Russian Il-2 Sturmovik ground-attack aircraft eastwards over Russian lines near Dünaburg, however he failed to return from this mission and is considered Missing in action since.

He was posthumously promoted to Major.

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Awards:

• Ehrenpokal der Luftwaffe on December 8, 1941
• Front Flying Clasp of the Luftwaffe for fighter pilots in Gold and Penant
• German Cross in Gold on 25 February 1942 as Leutnant in the 1./JG 54
• Iron Cross (1939)
• 2nd Class (7 September 1940)
• 1st Class (5 September 1941)
• Knight's Cross of the Iron Cross with Oak Leaves
• Knight's Cross on 16 April 1943 as Leutnant (war officer) and pilot in the I./JG 54
• 414th Oak Leaves on 2 March 1944 as Hauptmann (war officer) and Gruppenkommandeur of the I./JG 54