Process for the distillation of a mixture of isomeric diisocyanatodiphenyl-methanes

2,4′-MDI which contains very little 2,2′-MDI is prepared by distilling an isomeric starting mixture of diisocyanatodiphenylmethanes which includes 2,2′-diisocyanato-diphenylmethane, 2,4′-diisocyanatodiphenylmethane and 4,4′-diisocyanatodiphenyl-methane in at least one stage in a separating wall column. This distillation produces at least one mixture which contains 85 to 99 wt. % of 2,4′-diisocyanatodiphenylmethane, up to 15 wt. % of 4,4′-diisocyanatodiphenyl-methane and no more than 0.2 wt. % of 2,2′-diisocyanatodiphenylmethane.