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.