Preparation of polyisocyanates from polycarbamates

In the process of producing a diisocyanate comprising

• (a) condensing and rearranging formaldehyde and alkyl phenylcarbamate with the alkyl having 1 to 3 carbon atoms under acidic conditions to form alkyl methylene bis(phenylcarbamate) with the alkyl having 1 to 3 carbon atoms and
• (b) thermally decomposing the dialkyl methylene bis(phenylcarbamate) to diphenylmethane diisocyanate,
• the improvement comprises prior to step (b) treating the reaction product of (a) by first distilling the reaction product of (a) under reduced pressure at a temperature below 120.degree. C. until all solvent is removed and 65 to 85% of the unreacted alkyl phenylcarbamate present is removed, discontinuing the distillation, adding a polar solvent to undistilled distillation residue and selectively crystallizing and separating dialkyl methylene bis(phenylcarbamate) from the distillation residue.