Monitoring device for performing a measurement of a reaction liquid produced in a reaction tank in an NMR spectrometer , in particular for controlling a chemical reaction by means of NMR spectroscopy, the monitoring device having at least the following components: a hollow NMR sample probe for receiving the reaction liquid to be measured in the NMR spectrometer ; an inlet transport capillary for receiving the reaction liquid from the reaction tank and for transporting the reaction liquid from the reaction tank via a pump device toward the sample probe ; an outlet transport capillary for the return transport of the reaction liquid from the sample probe to the reaction tank ; a device (10, 10a - 10c) equipped for conducting temperature-control fluid around the inlet and outlet transport capillaries (105; 108), the device (10, 10a - 10c) for conducting temperature-control fluid around the inlet and outlet transport capillaries (105; 108) comprising tube portions , each of which is constructed in double-walled fashion from two radially closed partial tubes (19; 21, 21', 21") arranged nested in one another, comprising an outer partial tube and an inner partial tube (21, 21', 21"), the temperature-control fluid flowing between the outer and the inner partial tube (19; 21, 21', 21") and the inlet transport capillary or the outlet transport capillary being arranged radially within the inner partial tube (21, 21', 21"), and wherein the monitoring device has a closed temperature-control circuit for the temperature-control fluid , characterized in that the temperature-control circuit for the temperature-control fluid is geometrically designed such that in the intended operation the temperature-control fluid always flows only in a predetermined direction in each spatial region of the temperature-control circuit and no opposing flow of the temperature-control fluid in the immediate vicinity of the spatial region occurs. a hollow NMR sample probe for receiving the reaction liquid to be measured in the NMR spectrometer ; an inlet transport capillary for receiving the reaction liquid from the reaction tank and for transporting the reaction liquid from the reaction tank via a pump device toward the sample probe ; an outlet transport capillary for the return transport of the reaction liquid from the sample probe to the reaction tank ; a device (10, 10a - 10c) equipped for conducting temperature-control fluid around the inlet and outlet transport capillaries (105; 108), the device (10, 10a - 10c) for conducting temperature-control fluid around the inlet and outlet transport capillaries (105; 108) comprising tube portions , each of which is constructed in double-walled fashion from two radially closed partial tubes (19; 21, 21', 21") arranged nested in one another, comprising an outer partial tube and an inner partial tube (21, 21', 21"), the temperature-control fluid flowing between the outer and the inner partial tube (19; 21, 21', 21") and the inlet transport capillary or the outlet transport capillary being arranged radially within the inner partial tube (21, 21', 21"), and wherein the monitoring device has a closed temperature-control circuit for the temperature-control fluid , <EOS>