The bitumen modification through polyurethane prepolymers presents significant benefits for the manufacture of bituminous products for the paving industry. In this sense, this work explores the use, as bitumen modifier, of a novel reactive prepolymer synthesized by reaction of 4,4 ',diphenylmethane diisocyanate (MDI) and a vegetable oil-based polyol, castor oil (CO), previously transesterified with pentaerythritol. On the one hand, thermal analysis on transesterified CO revealed a highly stable prepolymer, making it suitable to be used for bitumen modification in a wide range of processing temperatures, 90-180 degrees C. On the other hand, the bitumen modification achieved for the polyurethane-modified strongly depends on the transesterification degree and post-treatment procedures involved. In this sense, Atomic Force Microscopy (AFM) proved that MDI-prepolymers, based on castor oil transesterified with 4 wt% pentaerythritol, lead to modified samples with a more complex microstnlcture which enhances rheological properties and reduces thermal susceptibility. In any case, viscoelastic and flow tests showed that the addition of small quantities of this reactive prepolymer to bitumen leads to products with enhanced performance at high in-service temperatures. (C) 2014 Elsevier Ltd. All rights reserved.