Process for preparing methylene diphenylene diisocyanate and optionally a mixture of methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate, comprising the steps of: α) providing a fraction comprising methylene diphenylene diisocyanate and secondary components with a proportion by mass of methylene diphenylene diisocyanate, based on its total mass and determined by gas chromatography, of more than 98.0%, optionally by α.1) separating methylene diphenylene diisocyanate and secondary components from a fraction comprising methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate to obtain (i) a polymethylene polyphenylene polyisocyanate-enriched mixture of methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate and (ii) the fraction comprising methylene diphenylene diisocyanate and secondary components with a proportion by mass of methylene diphenylene diisocyanate, based on its total mass and determined by gas chromatography, of more than 98.0%; β) purifying, comprising an isomer separation, of the fraction comprising methylene diphenylene diisocyanate and secondary components by distillation and/or crystallization in two or more, preferably in three to ten, more preferably in four to eight, partial steps (a, b,...) to obtain at least (i) two or more, preferably two to four, more preferably two to three, pure methylene diphenylene diisocyanate fractions (140-1, 140-2,...), each having a proportion by mass of methylene diphenylene diisocyanate, based on their total mass and determined by gas chromatography, of 99.9% or more and (ii) a secondary component fraction having a proportion by mass, based on its total mass and determined by gas chromatography, of methylene diphenylene diisocyanate in the range from 20.0% to 98.0%, characterized in that the secondary component fraction obtained in step β) is returned to one of the partial steps (a, b,...) of step β) in which none of the pure methylene diphenylene diisocyanate fractions (140-1, 140-2,...) from step β) is obtained in the form of distillate or crystallizate, and/or, if conducted, to step α.1). α) providing a fraction comprising methylene diphenylene diisocyanate and secondary components with a proportion by mass of methylene diphenylene diisocyanate, based on its total mass and determined by gas chromatography, of more than 98.0%, optionally by α.1) separating methylene diphenylene diisocyanate and secondary components from a fraction comprising methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate to obtain (i) a polymethylene polyphenylene polyisocyanate-enriched mixture of methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate and (ii) the fraction comprising methylene diphenylene diisocyanate and secondary components with a proportion by mass of methylene diphenylene diisocyanate, based on its total mass and determined by gas chromatography, of more than 98.0%; α.1) separating methylene diphenylene diisocyanate and secondary components from a fraction comprising methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate to obtain (i) a polymethylene polyphenylene polyisocyanate-enriched mixture of methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate and (ii) the fraction comprising methylene diphenylene diisocyanate and secondary components with a proportion by mass of methylene diphenylene diisocyanate, based on its total mass and determined by gas chromatography, of more than 98.0%; (i) a polymethylene polyphenylene polyisocyanate-enriched mixture of methylene diphenylene diisocyanate and polymethylene polyphenylene polyisocyanate and (ii) the fraction comprising methylene diphenylene diisocyanate and secondary components with a proportion by mass of methylene diphenylene diisocyanate, based on its total mass and determined by gas chromatography, of more than 98.0%; β) purifying, comprising an isomer separation, of the fraction comprising methylene diphenylene diisocyanate and secondary components by distillation and/or crystallization in two or more, preferably in three to ten, more preferably in four to eight, partial steps (a, b,...) to obtain at least (i) two or more, preferably two to four, more preferably two to three, pure methylene diphenylene diisocyanate fractions (140-1, 140-2,...), each having a proportion by mass of methylene diphenylene diisocyanate, based on their total mass and determined by gas chromatography, of 99.9% or more and (ii) a secondary component fraction having a proportion by mass, based on its total mass and determined by gas chromatography, of methylene diphenylene diisocyanate in the range from 20.0% to 98.0%, (i) two or more, preferably two to four, more preferably two to three, pure methylene diphenylene diisocyanate fractions (140-1, 140-2,...), each having a proportion by mass of methylene diphenylene diisocyanate, based on their total mass and determined by gas chromatography, of 99.9% or more and (ii) a secondary component fraction having a proportion by mass, based on its total mass and determined by gas chromatography, of methylene diphenylene diisocyanate in the range from 20.0% to 98.0%,