AbstractPolyurethane films were prepared by solution casting using a three‐component system consisting of polymeric MDI, a propylene oxide‐based polyether triol and kraft lignin fractions of different molecular weights. The polyether triol was used as soft segment. The composition of the polyurethanes was varied whereas the NCO/OH ratio was kept constant. The crosslink density increased with increasing molecular weight of the kraft lignin fraction used, which is a result of an increase in functionality of the kraft lignin with increasing molecular weight. Polyurethanes prepared from low molecular weight kraft lignin were found to be more flexible than those prepared from medium and high molecular weight kraft lignin (at lignin contents below 30%). At kraft lignin contents higher than about 30% (which at the NCO/OH ratio used corresponds to a hard segment content of about 70%), rigid and glassy polyurethanes were obtained regardless of the molecular weight of the kraft lignin fraction used.