PURPOSE: To condition and purify an org. isocyanate so that the isocyanate is made free from variation in its reactivity and excellent in quality by mixing the isocyanate with a specified silylated acid and carrying out treatment under specified conditions.

CONSTITUTION: An org. isocyanate such as 1,6-diisocyanatohexane is mixed with 0.001-1 mol.%, preferably 0.001-0.1 mol.% silylated acid of the formula based on the amt. of the isocyanate at 20-150°C. In the formula, X is the neutral acid residue obtd. by removing the acidic hydrogen atoms from an n-basic acid having a pKa value of at most 3, with the exception of hydrohalic acids, and (n) is an integer of 1-3. The silylated acid is, e.g. trimethylsilyl trifluoromethanesulfonate. The resultant mixture is optionally worked up by distillation after a residence time of at least 5 min. The org. isocyanate is conditioned and/or purified.