The difficulties associated with the use of optical pyrometry for temperature measurement in an electron‐beam furnace are discussed. Light from the filaments producing the electron beam may be scattered into the optical pyrometer. A procedure has been developed that allows a determination of the required correction to the optical pyrometer reading which will take into account the scattered‐light effect and also the error introduced by the correction. The problems associated with the use of thermocouples at high voltage in an electron‐beam furnace are also evaluated.