Modified polyisocyanates having an NCO group content of from 10 to 45% by weight, comprising a a) a polyisocyanate component consisting essentially of an organic polyisocyanate having an NCO group content of from 15 to 50% by weight which is liquid at room temperature, and b) 2 to 100% by weight, based on the weight of component a), of a polymer modifier consisting of at least one graft rubber which is dispersed in swollen form in component a), and a process for the production of the new modified polyisocyanates are described. The modified polyisocyanates are also useful in the production of plastics by the isocyanate polyaddition process.