A process for preparing 3(4),8(9)-bisformyltricyclo[5.2.1.02.6]decane by hydroformylating dicyclopentadiene with subsequent distillation wherein the hydroformylation of dicyclopentadiene is carried out in two stages, and, in the first hydroformylation stage, the reaction is effected 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 Period Table of the Elements to give 8(9)-bisformyltricyclo[5.2.1.02.6]dec-3-ene, and, in a second hydroformylation stage, the thus obtained 8(9)-bisformyltricyclo[5.2.1.02.6]dec-3-ene is converted, in homogeneous organic phase in the presence of transition metal compounds of group VIII of the Periodic Table of the Elements to 3(4),8(9)-bisformyltricyclo[5.2.1.02.6]decane.