Process for preparing a fraction of diisocyanates of diphenylmethane series containing at least 99% by weight of 2-ring methylenedi(phenyl isocyanate) based on the mass of the fraction, wherein a) aniline and formaldehyde are reacted in the presence of an acid catalyst to diamines and polyamines of the diphenylmethane series containing 2-ring methylenedi(phenylamine) and b) the diamines and polyamines of the diphenylmethane series containing 2-ring methylene di(phenylamine) are phosgenated, optionally in the presence of a solvent, giving a crude diisocyanate and polyisocyanate, and c) a fraction containing at least 98% by weight of 2-ring methylenedi(phenyl isocyanate) having a content of 4,4'-MDI of from 80 to 93.9% by weight, a content of 2,4'-MDI of from 6 to 19.9% by weight and a content of 2,2'-MDI of from 0.1 to 5% by weight, based on the mass of the fraction, is separated off from the crude diisocyanate and polyisocyanate and d) 4,4'-MDI is removed to an extent of from 10 to 98% from the fraction obtained in step c) and e) 2,2'-MDI is completely or partially separated off from the fraction obtained in step d), giving a fraction containing from 0 to 0.3% by weight of 2,2'-MDI, from 5 to 85% by weight of 4,4'-MDI and from 15 to 95% by weight of 2,4'-MDI, based on the mass of the MDI isomers, and, in step e), 50-99.9999% of 2,2'-MDI introduced with the crude diisocyanate and polyisocyanate into the separation in step c) and f) a fraction containing at least 99% by weight of 2-ring methylenedi(phenyl isocyanate), based on the mass of the fraction, and from 0 to 0.35% by weight of 2,2'-MDI, from 0.2 to 60% by weight of 4,4'-MDI and from 40 to 99.8% by weight of 2,4'-MDI, based on the mass of the MDI isomers, are separated off from the fraction containing from 0 to 0.3% by weight of 2,2'-MDI, from 5 to 85% by weight of 4,4'-MDI and from 15 to 95% by weight of 2,4'-MDI, based on the mass of the MDI isomers, and g) the 2,2'-MDI separated off in step e) is freed of low boilers by crystallization or distillation, giving an isomer mixture containing from 5 to 99.99% by weight of 2,2'-MDI, from 0 to 50% by weight of 4,4'-MDI and from 0.01 to 95% by weight of 2,4'-MDI based on the mass of the MDI isomers. a) aniline and formaldehyde are reacted in the presence of an acid catalyst to diamines and polyamines of the diphenylmethane series containing 2-ring methylenedi(phenylamine) and b) the diamines and polyamines of the diphenylmethane series containing 2-ring methylene di(phenylamine) are phosgenated, optionally in the presence of a solvent, giving a crude diisocyanate and polyisocyanate, and c) a fraction containing at least 98% by weight of 2-ring methylenedi(phenyl isocyanate) having a content of 4,4'-MDI of from 80 to 93.9% by weight, a content of 2,4'-MDI of from 6 to 19.9% by weight and a content of 2,2'-MDI of from 0.1 to 5% by weight, based on the mass of the fraction, is separated off from the crude diisocyanate and polyisocyanate and d) 4,4'-MDI is removed to an extent of from 10 to 98% from the fraction obtained in step c) and e) 2,2'-MDI is completely or partially separated off from the fraction obtained in step d), giving a fraction containing from 0 to 0.3% by weight of 2,2'-MDI, from 5 to 85% by weight of 4,4'-MDI and from 15 to 95% by weight of 2,4'-MDI, based on the mass of the MDI isomers, and, in step e), 50-99.9999% of 2,2'-MDI introduced with the crude diisocyanate and polyisocyanate into the separation in step c) and f) a fraction containing at least 99% by weight of 2-ring methylenedi(phenyl isocyanate), based on the mass of the fraction, and from 0 to 0.35% by weight of 2,2'-MDI, from 0.2 to 60% by weight of 4,4'-MDI and from 40 to 99.8% by weight of 2,4'-MDI, based on the mass of the MDI isomers, are separated off from the fraction containing from 0 to 0.3% by weight of 2,2'-MDI, from 5 to 85% by weight of 4,4'-MDI and from 15 to 95% by weight of 2,4'-MDI, based on the mass of the MDI isomers, and g) the 2,2'-MDI separated off in step e) is freed of low boilers by crystallization or distillation, giving an isomer mixture containing from 5 to 99.99% by weight of 2,2'-MDI, from 0 to 50% by weight of 4,4'-MDI and from 0.01 to 95% by weight of 2,4'-MDI based on the mass of the MDI isomers.