Abstract: The process comprises preparing a reaction mixture from a polyisocyanate component and from a polyol component which comprises one or more polyester polyols formed from at least one dimer fatty acid and/or at least one dimer fatty alcohol. In order to improve the processability of the polyurethane flexible foams so that these foams can be produced with the desired hardness and mechanical properties over a broad density range, the isocyanate reactive compounds contain, per 100 parts by weight thereof, 25 to 95 parts by weight of said polyester polyols and 5 to 75 parts by weight of one or more polyether polyols which are free of dimer fatty chains. In this way, no lipophilic filler is needed so that the fogging and emission properties of the foam can be improved. Moreover, softer foam can be produced without having to apply a too low NCO index resulting in better mechanical properties and in particular in a lower compression set.