A wide variety of carbide structures is described, as found in the surface layer of carburized thoriated‐tungsten filaments. Their origin is traced back to carburizing conditions and subsequent processing during tube making. A frequently occurring laminated carbide structure is found to contain less carbon than W2C. The thyratron relay method of carburizing control is critically reviewed. Abnormal filament current in manufactured tubes is explained as due to surface conditions which cause changes in thermal emissivity.