In situ Raman spectroscopy was used for studying the ternary 2% CrO3-6% V2O5/TiO2 catalyst, for which a synergistic effect between vanadia and chromia leads to enhanced catalytic performance for the selective catalytic reduction (SCR) of NO with NH3. The structural properties of this catalyst were studied under NH3/NO/O-2/N-2/SO2/H2O atmospheres at temperatures up to 400degreesC and major structural interactions between the surface chromia and vanadia species are observed. The effects of oxygen, ammonia, water vapor and sulfur dioxide presence on the in situ Raman spectra are presented and discussed.