The invention relates to a process for the preparation of mixtures of diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates (crude MDI) having a reduced iodine colour value by reacting the corresponding mixtures of diphenylmethanediamines and polyphenylpolymethylenepolyamines with phosgene in the presence of at least one inert organic solvent at an elevated temperature, wherein, after the end of the phosgenation, at least one monohydric or polyhydric polyoxyalkylene alcohol, preferably having a functionality of 2 to 3, and advantageously having a hydroxyl number of 20 to 1,800, or a mixture thereof is incorporated into the reaction mixture in an effective amount, advantageously in an amount of 0.1 to 10% by weight, based on the weight of the crude MDI, the phosgene and the inert organic solvent are then separated off, optionally up to 5% by weight, based on the weight of the crude MDI, of at least one phenol-based antioxidant and/or aryl phosphite are added to the reaction product and the reaction mixture is subjected to a thermal treatment.