the reactivity of polymethylene polyphenyl isocyanates containing 20 to 90 percent of methylenebis is improved , and stabilized against a decline on storage , by incorporating therein very small amounts of a compound of the formula ## str1 ## wherein r 1 = alkyl , aryl ; r 2 = alkyl ; r 3 = h , alkyl or aralkyl ; and r 1 and r 2 taken together represent the residue of an oxazoline or oxazine . 2 - ethyloxazoline is the preferred compound for incorporation in the polyisocyanate .