Connecting piece for connecting a reactor tube to a feed pipe (18, 35) configured as a thermal stress compensator for a reaction input stream and for lateral introduction of a multipoint thermocouple into the reactor tube, comprising the following constituent parts: (a) a first cylindrical or frustoconical section having a first end pointing towards a circular entry end of the reactor tube, wherein the first end of the connecting piece has the same internal diameter as the entry end of the reactor tube or as a tubular insulation piece introduced into the entry end of the reaction tube, and wherein the first section of the connecting piece is connectable to the entry end of the reactor tube in a gas-tight manner, (b) a second cylindrical or frustoconical section having a second end pointing towards a circular exit end of the feed pipe (18, 35), wherein the second end of the connecting piece has the same internal diameter as the exit end of the feed pipe (18, 35) and is connectable thereto in a gas-tight manner, (c) at least one transition piece connectable to the first and the second section of the connecting piece in a gas-tight manner, (d) wherein the constituent parts (a), (b) and (c) have a circular cross section and a longitudinal axis through the respective circle centre point, wherein the constituent parts are arranged one behind the other with their longitudinal axes in alignment and thus coincide on a common longitudinal axis, (e) a guide tube laterally attached to one of the constituent parts (a), (b) and (c), connected to the constituent part in a gas-tight manner and opening into that constituent part that at its end pointing away from the constituent part is provided with a fastening means which permits feedthrough and gas-tight fastening of a multipoint thermocouple, (f) wherein the angle between the common longitudinal axis and the longitudinal axis of the guide tube is between 15° and 60°, preferably between 20° and 45°, and the angle points in the direction of the feed pipe. (a) a first cylindrical or frustoconical section having a first end pointing towards a circular entry end of the reactor tube, wherein the first end of the connecting piece has the same internal diameter as the entry end of the reactor tube or as a tubular insulation piece introduced into the entry end of the reaction tube, and wherein the first section of the connecting piece is connectable to the entry end of the reactor tube in a gas-tight manner, (b) a second cylindrical or frustoconical section having a second end pointing towards a circular exit end of the feed pipe (18, 35), wherein the second end of the connecting piece has the same internal diameter as the exit end of the feed pipe (18, 35) and is connectable thereto in a gas-tight manner, (c) at least one transition piece connectable to the first and the second section of the connecting piece in a gas-tight manner, (d) wherein the constituent parts (a), (b) and (c) have a circular cross section and a longitudinal axis through the respective circle centre point, wherein the constituent parts are arranged one behind the other with their longitudinal axes in alignment and thus coincide on a common longitudinal axis, (e) a guide tube laterally attached to one of the constituent parts (a), (b) and (c), connected to the constituent part in a gas-tight manner and opening into that constituent part that at its end pointing away from the constituent part is provided with a fastening means which permits feedthrough and gas-tight fastening of a multipoint thermocouple, (f) wherein the angle between the common longitudinal axis and the longitudinal axis of the guide tube is between 15° and 60°, preferably between 20° and 45°, and the angle points in the direction of the feed pipe.