We describe a microchemical reactor built by silicon processing and metal deposition techniques that enables efficient and safe direct fluorination of toluene, a highly exothermic process difficult to implement conventionally on a macroscopic scale. Gas and liquid reagents were contacted cocurrently at room temperature in the microfabricated reactor, and gas-liquid distribution patterns were characterized. A flow regime map, containing slug and annular-dry flows, was obtained for liquid velocities relevant to gas-liquid reactions in microchemical systems. During annular-dry flow operation, the substrate conversion and product distribution were studied as a function of the operating conditions: toluene concentration, fluorine-to-toluene molar ratio, solvent type, and quenching conditions. Among the solvents tested, including acetonitrile, methanol, 1,1,2-trichloro-1,2,2-trifluoroethane, and octafluorotoluene, the highest selectivities toward ring fluorination were obtained in acetonitrile. At toluene conversions of 58%, a combined selectivity of ortho-, meta-, and para-fluorotoluenes of up to 24% was obtained.