Pure and K-doped vanadia/titania catalysts have been studied by transient response method and in situ DRIFTS during toluene partial oxidn. They were characterized by FT-Raman spectroscopy, TPR, TPD of pyridine and XPS. In order to elucidate the role of different vanadia species for catalysis, the study was carried out in aerobic and anaerobic conditions. Monolayer species (monomeric and polymeric) are responsible for the catalytic activity and selectivity. Bulk V2O5 present in the catalysts with high concn. of vanadia supplies oxygen for the monolayer species easily reducible by toluene. This oxygen is necessary for the products formation and desorption in the absence of gaseous oxygen. Potassium at low concn. diminishes the total rate of toluene oxidn. and suppresses deactivation provided by cocking. This is due to transformation of the polymeric species to the not active K-doped monomeric ones. [on SciFinder (R)]