The invention relates to a process for the production of di- and polyisocyanates of the diphenylmethane series and mixtures thereof, in which aniline and formaldehyde are reacted in the presence of an acid catalyst to form di- and polyamines of the diphenylmethane series; the di- and polyamines of the diphenylmethane series are phosgenated, optionally in the presence of a solvent, to obtain a crude di- and polyisocyanate; from the crude di- and polyisocyanate, at least one fraction containing at least 5 wt. % 3-ring and multi-ring MDI and at least one fraction containing at least 95 wt. % 2-ring MDI are separated by distillation, and the acidities of the fraction containing at least 5 wt. % 3-ring MDI and multi-ring MDI, and/or the fraction containing at least 95 wt. % 2-ring MDI are determined analytically; and an acidic compound is added to the fraction containing at least 5 wt. % 3-ring MDI and multi-ring MDI, and/or the fraction containing at least 95 wt. % 2-ring MDI, so that the desired acidity is achieved.