The behaviour of the regioselectively generated carbocation centers at C(2) and C(6) in 1,2-trimethylenenorbornanes was investigated in order to study the occurrence or absence o f a degenerate rearrangement EF?M in the adamantane rearrangement of both 1,Zendo-(1) and 1,2-exo-trimethylenenorbornane (2) to 2-endo,6-endo-trimethylenenorbornane (3). A degenerate rearrangement E e M is inevitably involved inasmuch as a 1,2-trimethylenenorborn-2-yl cation E not only is formed directly as manifested by the conversions of the reactants 4 (C(2), C(3)-olefin) and 6 (C(2), C(3')-olefin), but also indirectly (via F-E) if the leaving group at C(6) to be ionized occupies the endo-position (dendo-alcohol 8). No degenerate rearrangement E@M is operative starting from reactants that lead directly to a 2,6-trimethylenenorborn-2-y1 cation G ; this is the case with both the ionization of the 6-exo-alcohol 10 having the leaving OH-group in a stereoelectronically favoured configuration to undergo simultaneous C(I),C(2)-bond migration (-G) as well as the protonation of the olefin 13 which is followed by the same reaction pathway.1. Introduction. -In the adamantane rearrangement of both 1,a-endo-(l),) and 1,Zexo -trimethylenenorbornane (2)2) to 2-endo,6-endo-trimethylenenorbornane (3)*), a degenerate rearrangement (EsM3); Scheme 2) is involved as verified from treatment of D-labelled 1 ,2-endo -, 1,2-exo-, and 2-endo,6-endo -trimethylenenorbornane (D-labelled 1,