Time-resolved in situ IR was performed during selective catalytic reduction of NO with NH3 on supported V2O5-WO3/TiO2 catalysts to examine the distribution and reactivity of surface ammonia species on Lewis and Bronsted acid sites. While both species were found to participate in the SCR reaction, their relative population depends on the coverage of the surface vanadia and tungsta sites, temperature, and moisture. Although the more abundant surface NH4,ads+ intermediates dominate the overall SCR reaction, especially for hydrothermally aged catalysts, the minority, surface NH3,ads intermediates exhibit a higher specific SCR activity. (TOF). The current study serves to resolve the long-standing controversy about the active sites for SCR of NO with NH3 by supported V2O5-WO3/TiO2 catalysts.