Connecting piece for connecting a reactor tube to a feed pipe 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 , wherein the second end of the connecting piece has the same internal diameter as the exit end of the feed pipe 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 , wherein the second end of the connecting piece has the same internal diameter as the exit end of the feed pipe 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 . <EOS>