The invention relates to a process for preparing formic acid by reaction of carbon dioxide with hydrogen in a hydrogenation reactor in the presence of a transition metal complex as a catalyst comprising at least one element from group 8, 9 or 10 of the Periodic Table and at least one phosphine ligand with at least one organic radical having at least 13 carbon atoms, of a tertiary amine and of a polar solvent to form a formic acid-amine, adduct, which is subsequently dissociated thermally to formic acid and the corresponding tertiary amine. on unit.