Abstract The mechanism of propene elimination from metastable methyleneimmonium ions is discussed. The first field-free region fragmentations of complete sets of isotopically labelled methyleneimmonium ions (H 2 C= equation image+ R 1 R 2: R 1= R 2= nC 3 H 7; R 1= R 2= iC 3 H 7; R 1= nC 3 H 7; R 2= C 2 H 5; R 1= nC 3 H 7; R 2= CH 3; R 1= nC 3 H 7; R 2= H) were used to support the mechanism presented. The relative amounts of H/D ...