Abstract: Modified polyisocyanates with biuret structure containing terminal urethane groups and having general formula: ##STR1## in which R.sub.1 is an aliphatic, cycloaliphatic or aliphatic-cycloaliphatic radical, simple or substituted, and R.sub.2 can have the same meanings as R.sub.1 or can be the radical R.sub.3 (OC.sub.n H.sub.2n).sub.x --.Process for the preparation of modified polyisocyanates having biuret structure containing terminal urethane groups having general formula (I) consisting in reacting an organic diisocyanate with water or with a compound which splits off water, in the presence of a compatibilizing agent having a hydrogen atom reactive towards the isocyanate group, as it can be determined by Zerewitinoff's method.