We have evaluated the mixing performance of a microreactor with inline ultraviolet (UV) and Fourier transform-infrared (FT-IR) measurement. We performed the well-known Villermaux-Dushman reaction with inline and offline UV measurements. Absorbance was continuously reproduced at each flow rate with inline measurement. The difference between the inline and offline measurement results was within +/- 5% at faster flow rates than 2 mL/min. However, the difference between the inline and offline measurement results at 1 mL/min occurred because the sample changed until the start of offline measurement. It is shown that "real-time" monitoring and more accurate analyses were performed with this inline measurement system. We have also performed the methanol-ethanol mixing process with inline FT-IR measurement. The absolute slope of absorbance peak height with changing from methanol to a mixing solution became larger as the flow rate increased, while it hardly changed at faster flow rates than 10 mL/min (1.0x10(-5) m(3)/min). This suggests that the mixing performance improved as the flow rate increased, while the mixing performance was saturated at faster flow rates. The tendency in the mixing performance was in good agreement with that obtained from the Villermaux-Dushman reaction. Therefore, it has been confirmed that the mixing performance of microreactors could be appropriately evaluated with inline UV or FT-IR measurement.