Sulfur-containing deposits form on a monolithic V₂O₅-WO₃/TiO₂ (VWT) catalyst during SCR-DeNO_x with NH₃ at 473 and 523 K and pressures up to 500 kPa in the presence of SO₂ with sulfate contents of 1.7 to 13.0 wt%. Using thermogravimetric analysis and diffuse reflectance infrared spectroscopy, these deposits are determined to be mainly NH₄HSO₄ for SCR temperatures > 523 K. At lower temperatures, (NH₄)₂SO₄ is formed. The thermal stability of NH₄HSO₄ supported on different transition metal oxides including V₂O₅, WO₃, TiO₂, MoO₃, and Al₂O₃ varies with decomposition temperatures from 620 to 820 K. Using DFT calculations, it is shown that the thermal stability of supported NH₄HSO₄ is mainly determined by hydrogen bonding of the HSO₄ ⁻ anions with the metal oxide surface. Increasing electronegativity of the metal atoms of the support oxide leads to weakening of the S-O bonds in the HSO₄ ⁻ anions and to lower decomposition temperatures of the supported NH₄HSO₄.