Despite the close association between tellurium (Te) and gold (Au) in many epithermal, orogenic, and intrusion-related hydrothermal ore deposits, the hydrothermal chemistry of Te remains poorly understood. We studied the protonation/deprotonation and structure of tellurous acid (H2TeIVO3) species in aqueous solutions and in water vapor as a function of pH from room-temperature to 505°C using a number of methods: TeO2(s) solubility in solution and steam, potentiometry, NMR spectroscopy, and in situ XAS spectroscopy.The solubility of TeO2(s) increases by up to a factor 80 between ambient temperature and 200°C. As the temperature increases, the pH range over which the neutral species H2TeO3(aq) dominates to the detriment of the ionic species H3TeO3+, HTeO3- and TeO32- expands. The structure of these species is a trigonal pyramid with the Te atom at its apex, indicating a stereochemically active electron pair. The Te-O bond lengths increase with increasing protonation (i.e., decreasing pH).Although hydrated tellurite species such as TeO2(H2O)(g) and TeO2(H2O)2(g) exist in significant concentrations in vapors equilibrated with TeO2(s), these species are unlikely to play a significant role in natural systems, because of the high solubility of Te(IV) in the liquid phase under these conditions. Solubility calculations conducted with the new and existing properties confirm the importance of reduced species for the vapor transport of Te, with a partitioning coefficient (Kd=Te concentration in vapor/Te concentration in liquid) up to >105 in favor of the vapor, and ppm concentrations of Te in reduced vapors at 300°C.Thermodynamic calculations show that slightly basic, mildly reduced fluids that can transport Au efficiently as Au(HS)2- can also carry significant Te (e.g., ~100ppb at 300°C). The calculations also suggest that under magmatic hydrothermal conditions, large amounts of Te can be transported as Te(IV) complexes in oxidized fluids (coexisting with SO2(g)). Reduction of Te(IV) caused by processes such as fluid-rock interaction or fluid mixing will lead to a dramatic decrease in the solubilities of both Te and Au, and to the precipitation of telluride minerals.