Spectral and total emissivity measurements of thoria‐coated tungsten filaments show that irreversible and reversible changes of the emissivity occur. &egr;&lgr; (&lgr;=0.65&mgr;) varies from 0.2 to 0.7 and &egr;tfrom 0.2 to 0.5.Certain variations of the thermal radiation can be related to changes of thermionic emission.Passage of current through the coating affects emissivity and thermionic emission. This is shown by measuring infra‐red radiation and electron current simultaneously. The existing relationship between emissivity and thermionic activation is discussed.