Due to the slow kinetics observed in batch reactions and the mixture of SO2FCl and SO2F2 obtained, we turned to flow technology as a tool for more effectively generating and controlling the delivery of this reactive gas . Hereto, we designed a modular system (Figure ) using microfluidic technology to greatly enhance the safety and scalability of the overall process (25). Our modular system consists of two interconnected flow reactors. The first reactor is a packed-bed reactor filled with KF that generates SO2F2 on demand via chlorine-fluorine exchange. The second reactor is where the generated gaseous SO2F2 is mixed with the nucleophilic partner, ultimately yielding the desired SuFEx product. Since the first reactor generates SO2F2 on demand, the reagent remains contained and is subsequently mixed with the nucleophilic partner in the second reactor. By immediately reacting away the toxic SO2F2 in the SuFEx module, our modular system effectively eliminates the safety and practical concerns associated with the handling of this reagent, while generating only the required quantities. In addition, we anticipated that our use of flow technology would also reduce the time required for halogen substitution reactions, thanks to enhanced liquid-to-solid contact in the first Cl-F exchange module and excellent gas-to-liquid mass transfer in the SuFEx module . Our experiments confirmed the effectiveness of our modular system: when we directed a solution of sulfuryl chloride over the packed-bed reactor filled with a mixture of KF and glass beads (Figure , see Supplementary Materials for further details), we observed a rapid and selective formation of SO2F2. We found that varying the flow rate was crucial for the selectivity of the transformation, and that 0.5 mL/min was the optimal flow rate in terms of conversion, selectivity, and time (7 min reaction time), effectively avoiding the presence of undesired SO2FCl. Under optimized conditions, the packed-bed reactor was able to produce ~18 mmol of SO2F2 starting from a ~80 mmol KF bed (see Supplementary Materials for further details). Having obtained promising results from our SO2F2 generator, we proceeded to integrate it with the SuFEx module to enable the reaction with nucleophilic partners. By introducing the appropriate nucleophiles with an excess of a base, we were able to obtain a diverse range of SuFExed products with excellent isolated yields in just two minutes of residence time (Figure ). This short residence time can be attributed to the intimate contact between gas and liquid phase in flow (i.e., enhanced gas-liquid mass transfer) , which should allow for the generation of large libraries of SuFExed compounds with minimal effort and time. Notably, a variety of phenols could be cleanly converted to their corresponding fluorosulfates regardless of the position or electronic nature of the substituents (Figure , compounds 1-6). While using large quantities of gaseous SO2F2 in batch reactions can be challenging, our flow protocol overcomes this issue, allowing for a gram-scale synthesis of fluorosulfate 1 by simply increasing the amount of starting materials pumped through the reactor assembly .