Abstract: A reactor tube of a microreactor assembly including a first central temperature sensor and a channel formed in its outer surface to circulate a coolant. A tightly fitting conductive core surrounds the cooling channel and is sealed to the reactor tube at both ends to confine the coolant except for inlet and outlet holes. Around that core a metallic tube is fitted having a longitudinal slot to which a second temperature sensor is fixed. A heating element to heat the tube and the reactor tube is disposed on the slotted tube and second sensor so that temperature changes on the outer wall of the reactor tube-core sub-assembly can be detected. Both the first and second temperature sensors are adapted to feed output signals to a controlling data processing unit.