Disclosed is a thermionic cathode for electron tubes comprising a material that has a substrate, a compound of an element forming an emitting monolayer, chosen from among the rare earth zirconates, rare earth hafnates, rare earth aluminates and rare earth berylates, and a reducing agent which, at the working temperature of the cathode, reacts with the compound releasing the element that forms the monolayer. Application in particular to electron grid tubes. FIG. 1.