A multitubular fixed bed reactor suitable for carrying out a catalytic process is described. There actor includes a plurality of reactor tubes, one or more of which include a fixed bed of catalyst and is/are at least partially surrounded by a heat transfer medium, preferably a cooling medium, wherein the one or more reactor tubes include at least one insert, especially a heat transfer insert and/or a pressure drop decreasing insert. Preferably the tubes are elongate, and preferably the inserts are elongate. The inserts may not extend the full length of their respective tube or tubes. The inserts may extend partly, substantially or wholly along the length of the relevant tube(s). All the tubes of the reactor may include a heat transfer and insert. Most preferably, the insert of the present invention reduces temperature differentials in a reactor tube and decrease the pressure drop over the reactor tube, so as to minimise even temperature differences, and thus optimise reaction conditions and catalytic activity within the reactor tube.