The present invention relates to an electrocatalytic system for the joint treatment of oxidizing polluting entities of the NOx type and reducing polluting entities of the hydrocarbons HC or CO type present in a gas to be purified, in particular an exhaust gas resulting from a combustion engine, said system comprising: a catalyst A for reduction of the polluting entities of the NOx type, a catalyst B for oxidation of the polluting entities of the hydrocarbons HC type and CO, a compound E which conducts electrons and ions by oxide ions, said catalysts A and B being in contact with the compound E, said compound E being composed of an oxide which conducts ions and electrons corresponding to the molar formulation: Ce1-y-zO2-xMyNz in which Ce is cerium, M is an element chosen from: Gd, Y, Sc, Sm, La, Pr, Nd, Er or Tb, y is between 0.01 and 0.4, N is an element having several degrees of valency chosen from: Ti, V, Cr, Mn, Fe, Co, Ni or Cu, z is less than 0.4 and x is greater than 0.05.