A reactor is provided for the preparation of modified bitumen, which reactor comprises a horizontal housing comprising a cylindrical wall and two side walls, wherein a bitumen inlet has been provided at or near one of the side walls of the housing and a bitumen product outlet has been provided at or near the opposite side wall of the housing, wherein a plurality of inlets for the provision of oxygen-containing gas has been provided in the cylindrical wall of the housing between the bitumen inlet and the bitumen product outlet, which multi-purpose reactor is further provided with a mixer arranged inside the housing comprising at least one rotor rotating within at least one stator having a plurality of openings. Also there is provided a process for the preparation of modified bitumen, which comprises contacting bitumen at elevated temperature and pressure with a modifier in a reactor as herein described.