Since the discovery of the colloidal perovskite nanocrystals three years ago, they have rapidly grown to become one of the most promising classes of nanomaterials for large-scale applications in optoelectronic devices. Anion exchange reactions of the highly luminescent cesium lead halide perovskites (CLHPs) provide a facile post-synthetic route for the tuning of the absorption/emission band-gap of CLHPs. These post-synthetic reactions allow the utilization of CLHPs in various optoelectronic applications including third-generation photovoltaic cells and light emitting diodes. Studies of anion-exchange reactions are typically conducted using the time- and material-intensive flask-based synthesis approach. Batch scale synthesis strategies are notorious due to (a) batch-to-batch variation, (b) inefficient and irreproducible mixing timescales, (c) manual sampling and characterization at room temperature, and (d) poor size distribution of the resulting nanocrystals after scale-up. Here, we present a modular multiphase microfluidic strategy with an in situ spectral monitoring capability that enables the systematic kinetic study of anion-exchange reactions of CLHP nanocrystals. Utilizing the microfluidic nanocrystal synthesis platform, we monitor absorption and emission spectra of CLHPs, in real-time, over residence times ranging between 100 ms and 17 min. In-situ monitoring of the optoelectronic properties of CLHPs over different synthesis conditions enables fundamental and applied studies of structural tuning of CLHPs via anion-exchange reactions. The enhanced mixing feature of the multiphase flow along with the novel anion-exchange framework using ZnX2 (X=I or Cl) facilitates on-demand bandgap tuning of high-quality CLHPs (i.e., narrow size distribution with high quantum yield) via a positive feedback loop in which synthesis parameters are varied until the target optoelectronic characteristics are achieved.