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 bond.