The conformation of a tricyclo[5,3,1,1^2,6]dodecane derivative has been determined by a single-crystal X-ray analysis. The crystals are monoclinic, space groups P2₁/c with four molecules of C₂₀H₂₅IO₃ in a unit cell of dimensions a= 10·36, b= 11·62, c= 15·46 Å, β= 104° 10′. The structure was solved by the heavy-atom method and refined by least-squares methods. R is 0·136 for 2904 data. The tricyclododecyl system adopts a distorted double-chair conformation.