We successfully developed an in situ UV-VIS absorption spectroscopic flow reactor system for investigation of solid-liquid two-phase reactions within the second time scale at high temperatures (solid-state-catalyst mediated hydrothermal flow reactor system, SSCM-HF). The SSCM-HF consisted of a water reservoir, high-pressure pump, sample injector, high-temperature reactor involving solid-state catalysts, a cooling bath, a pressure regulator, and a temperature controller. The high-temperature reactor contains a reactor column having micrometer sized solid particles, and is connected to an optical window using transparent fused-silica capillary tubing. Liquid samples are exposed for the second time scale, which can be adjusted by changing the flow rate. In situ monitoring of reactions using SSCM-HF was performed at 200-1114 nm within 7.36-117.79 s at temperatures up to 300 degrees C and pressures up to 30 MPa. We demonstrated the usefulness of SSCM-HF by inspecting 5'-hydroxymethylfurfural (HMF) formation from glucose and cellulose. The influence of metal oxides, such as WO3 and Nb2O5, was investigated using both a conventional batch reactor and the SSCM-HF. The simple metal oxides showed catalytic activities for the dehydration of glucose to HMF. The SSCM-HF demonstrated in situ detection of a small amount of HMF in the presence of WO3 deposited solid particles within 29.4 s at 200 degrees C. This study demonstrates the application of the SSCM-HF as a new research tool for screening and investigating solid-state catalysts in detail in liquid-phase reactions at high temperatures. (C) 2016 Elsevier B.V. All rights reserved.