A series of reactions between the technetium tricarbonyl core, [99mTc(CO)3(OH2)3]+ and both the bifunctional chelate dithiazole valeric acid (DTV) and insulin derivatized with DTV were performed using microfluidic, microwave, and conventional labeling methods. At low concentrations of ligand, the microfluidic reactor resulted in higher yields than both the microwave and conventional reactions. The labeling of DTV at a concentration of 0.01 mg/ml (32.2 µm) and 100 °C did not occur using conventional techniques, whereas the yield after 7.85 min was 61% in the microfluidic reactor and 18% in the microwave reactor. The labeling of a DTV–insulin conjugate (2.1 mg/ml, 330 µm) at 37 °C was conducted using conventional methods producing the desired product in 21% yield in 15.7 min compared with 40% of the desired product in the identically formulated microfluidic reactor. In addition to the higher radiochemical yield, the radiochemical purity was significantly improved in the microfluidic reactor. The microfluidic reactor offers a number of advantages over conventional and microwave methods and is worthy of further exploration as a method to prepare molecular imaging probes derived from Tc-99m. Copyright © 2011 John Wiley & Sons, Ltd.