Composite polymers, especially composite polyols containing active hydrogen atoms can be prepared by polymerizing one or more olefinically unsaturated compounds in the presence of a liquid polymer containing active hydrogen atoms and a modified initiator; the modification comprising the reaction between a liquid polymer containing one or more groups which can react with isocyanato groups and a compound containing two or more isocyanato groups which has been partially converted by reacting it with an organic hydroperoxide or a free-radical initiator also containing additional active hydrogen atoms. The composite polymers exhibit a stable dispersed polymer phase and can be suitably applied as starting materials for polyurethane foams.