The evolution of the electron emission of impregnated cathodes has been measured over many thousands of hours of operation at various temperatures in diodes and traveling‐wave tubes. Certain physical methods were used to investigate the correlation between these results and the mechanisms of cathode operation: X‐ray fluorescence electron microprobe analysis has given a new access to the study of the consumption of the active materials (barium and calcium) at various stages of cathode life. While barium depletes according to thet1/2law, an approximatet1/3law found for calcium could result from a two‐step process in the barium and calcium chemistry. The formation and desorption of synthetized emissive layers on a nonimpregnated porous tungsten plug in a Becker‐type test diode is one way to investigate the properties of emissive surfaces. The behavior of layers synthetized at low temperatures onto active impregnated cathodes suggests a surface equilibrium depending on the chemical nature of the substrate. Emission charts are proposed to compare, over a large range of temperatures, the different types of cathodes, in particular, the characteristics of uncoated and coated ones.