V2O5-WO3/TiO2 catalysts were prepared by impregnation method and in situ electrical conductivity measurements were carried out to investigate the reaction mechanism for ammonia SCR (selective catalytic reduction) of NOx.The electrical conductivity change with ammonia supply and the increase of electrical conductivity were mainly caused by reduction of the labile surface oxygen.The electrical conductivity change of catalysts shows close relationship with the conversion rate of NOx.Variation of conversion rate in atmosphere without gaseous oxygen also supports that the labile lattice oxygen is indispensable in the initial stage of the de NOx reaction.These results suggest that the liable lattice oxygen acts decisive role in the de NOx mechanism.They also support that De NOx reaction occurs through the Eley-Rideal type mechanism.The amount of labile oxygen can be estimated from the measurement of electrical conductivity change for catalysts with ammonia supply.