Abstract The addition of phenyl (trichloromethyl) mercury to five substituted norbornenes is described, and the results compared with those obtained with bicyclo [2.2. 2] oct-2-ene and 3, 3, 5, 5-tetramethylcyclopentene. The reaction proves to be useful for synthetic purposes, in that it leads usually to ring expansion. However, a possible disadvantage is that the addition step is sensitive to steric hindrance and depends on the reactivity of the double ...