A process for selectively hydroformylating dicyclopentadiene to 8(9)-formyltricyclo-[5.2.1.02,6]dec-3-ene in a heterogeneous reaction system using an aqueous solution of transition metal compounds, containing water-soluble organic phosphorus(III) compounds in complex-bound form, of group VIII of the Periodic Table of the Elements, wherein the water-soluble organic phosphorus(III) compounds are alkali metal or alkaline earth metal salts of sulfonated arylphosphines and aryldiphosphines.