Isomerisation of endo-tetrahydrodicyclopentadiene to exo-tetrahydrodicyclopentadiene has been studied. A conversion of 80% with 100% selectivity of exotetrahydrodicyclopentadiene has been obtained at 70 degrees C using mol ratio of conc. H2SO4 to endo-tetrahydrodicyclopentadiene as 4.08:1.475 (i.e. 2:1 by weight). A kinetic study of the isomerisation process has also been carried out.