Monolithic microfluidic reactors for the safe, expedient, and continuous synthesis of products involving unstable intermediates were fabricated and assessed. The formation of diazonium salts in anhydrous conditions and their subsequent in situ chlorination within microfluidic channels under hydrodynamic pumping regimes is presented. Significant enhancements in yield were observed due to enhanced heat and mass transfer in microfluidic systems. Analysis performed using off-line GC and GC−MS was compared with on-line, on-chip Raman spectroscopy for the direct determination of analytes.