Increase in traffic volume has led to a wider use of Polymer modified Bitumens (PmB's) in road construction. Although the mechanical properties of such materials have been widely studied, their change with time in service, also called ageing, is yet to be fully understood. One of the most important issues is to identify the process involved: is PmB ageing a consequence of bitumen ageing, polymer ageing or both at the same time? Moreover, most PmB's feature a two-phase structure made of polymer rich areas along with polymer poor regions, depending on the bitumen chemistry, the polymer nature and content. It is therefore important to take this peculiar structure into consideration when trying to sort out the respective effect on ageing of the polymer and the bitumen. Infrared microscopy allows characterizing separately different phases in heterogeneous products; therefore it is appropriate for PmB's. In this paper, PmB's were studied in their original state and after conventional tests claimed to simulate the ageing during the mixing process and several years of road service (RTFOT + PAV). The PmB's included plastomers and elastomers, some of them being in situ crosslinked. Infrared microscopy was used to determine for each phase the polymer rate and functional indices characterizing the bitumen such as aromaticity, aliphaticity and condensation, and also to map the polymer distribution in the PmB. The characterization of PmB in their original state points out which species of the bitumen are involved in the polymer swelling and the effect of the polymer nature. The characterization of the same PmB's after the RTFOT + PAV ageing shows how the bitumen species responsible for the swelling evolve during ageing. In addition, kinetic studies were performed using an heating cell fitted to the IR microscope. They confirmed the tendencies obtained with the conventional ageing tests. These studies come to the conclusion of an interdependence of the ageing of the different constitutive phases in a PmB and of chemical exchanges between them. They make clear the micro-morphological modification induced by ageing in a PmB. They finally help to better understand the effect of a crosslinking on the PmB microstructure and its ageing mechanism. (c) 2007 Elsevier Ltd. All rights reserved.