Metal dusting behaviour of alloy800H was investigated in a laboratory-pressurized reactor where CO-H2-CO2-H2O gas mixtures flowed continuously, simulating the gas environments in actual syngas production plant. Four test conditions, having a variety of gas pressures and H2O contents, were conducted for a long rod specimen that was imposed thermal gradient of 777°C to 454°C in a longitudinal direction. After an exposure of 100h at high pressure, metal dusting has initiated with a bell-shape distribution on the test specimens at temperatures between 600°C and 700°C. Gas pressure has accelerated metal dusting even if the gas atmospheres had the same gas composition. On the contrary, metal dusting has been alleviated with increasing the H2O content in the test gas. Oxide scale characterisations were analyzed by using Raman spectroscopy and XPS at the test specimen surfaces exposed at different temperature. The ratio of a protective Cr2O3 scale to spinel oxides such as FeCr2O4 and MnCr2O4 has increased with increasing the exposure temperature, with reducing the gas pressure, and with lowering the H2O content. These behaviours have been interpreted from thermodynamic indices: carbon activity ac and the oxide scale integrity related to oxygen potential Po2 as a function of temperature.