Process for preparing tricyclodecanedialdehyde by hydroformylation of dicyclopentadiene by means of a CO/H.sub.2 mixture at elevated temperature and under superatmospheric pressure in the presence of a rhodium catalyst which has not been modified by means of a ligand and is homogeneously dissolved in the hydroformylation medium, wherein the hydroformylation is carried out at a pressure of from 200 to 350 bar in at least two reaction zones, with a reaction temperature of from 80 to 120.degree. C. being set in a first reaction zone and a reaction temperature of from 120 to 150.degree. C. being set in a reaction zone following this reaction zone, with the proviso that the reaction temperature in the subsequent reaction zone is at least 5.degree. C. higher than in the preceding reaction zone.