Tungsten (VI) speciation in hydrothermal solutions is investigated through in-situ Raman spectroscopy coupled with the fused silica glass capillary technique at temperatures up to 400 °C. The effect of temperature, pH, chlorinity and carbonate speciation are evaluated. At all investigated temperatures, the tungstate ion WO4 2-(927 cm-1) is the only W species in solution at pH > 10. At a given pH, the presence of dissolved carbonates and chloride does not affect the tungsten speciation. Tungsten polymers remain stable up to 400 °C under acidic to circum-neutral pH conditions and total tungsten concentration above 0.01 mol kgH2O-1. Among the three observed polymers, namely [W7O24] 6-(paratungstate-A, ~ 960 cm-1), [W10O32] 4-(tungstate-Y, ~ 970 cm-1), and α-[H2W12O40] 6-(α-metatungstate, ~ 990 cm-1), only the hepta-and dodeca-tungstate are stable at elevated temperature. Combined with revised literature data, these results allow the thermodynamic stability constants of these W polymers to be constrained, enabling quantitative predictions of their relative abundance at temperatures up to 300 °C. These predictions suggest that W polymerization occurs under hydrothermal conditions even at low W concentration (down to 10-5 mol•kgH2O-1) under acidic conditions. These observations imply that the currently available geochemical models on W transport and deposition in deep and hot geological fluids need to be revised.