Small addition of potassium to V/Ti-oxide catalyst (K:V = 0.19), consisting of 3.7 monolayer VOx, increased activity and selectivity in partial oxidation of toluene. In order to elucidate the nature of vanadia species formed on the surface of V/Ti-oxide upon potassium doping, the catalysts were studied by transient kinetics method. The transient product responses during toluene oxidation by the oxygen present in the catalyst were compared for K-doped and non-doped samples. The formation of CO2 decreased and formation of benzaldehyde increased with addition of potassium. This suggests a lower surface concentration of electrophilic oxygen (O-, O-2(-)), which is usually responsible for the deep oxidation, and a higher concentration of nucleophilic oxygen (O-2(-)), responsible for the partial oxidation. The catalysts were characterised by means of HRTEM, FT-Raman spectroscopy and V-51 NMR. Potassium addition introduces a disorder in the crystalline structure of hulk V2O5 particles resulting in better spreading of V2O5 over TiO2 surface. The interaction of V2O5 with TiO2 was facilitated upon K-doping, leading to the increased formation of monomeric vanadia species, which are the active sites in toluene partial oxidation to benzaldehyde.