Biomass SCWG is seen as one of the most promising thermochemical pathways to produce green hydrogen. However, there is a serious concern about corrosion of biomass SCWG reactor alloys for long-term operation due to the presence of high-pressure and high- temperature supercritical water, aggressive catalyst and reducing gaseous products (such as H2 and CO). In this study, the corrosion performance of UNS N06625, a commonly applied Ni-based alloy for high temperature systems, was investigated in the conditions associated with the SCWG of three representative biomass model compounds (including cellulose, lignin, and xylan) at 500 °C and 30 MPa for total 12 cycles of batch-mode operations. The produced gaseous mixtures were collected and analyzed using Micro-GC to explore the environmental chemistry. Except for weight change measurements, the corrosion products formed on the alloys were also characterized using SEM, EDS and XRD techniques to advance fundamental understanding of how the alloy corrodes under the SCWG conditions. Additional investigations, including cross-sectional SEM/EDS analysis and weight loss measurement following scale removal will be pursed in subsequent research to gain quantitative understanding of corrosion mechanisms.