Cobalt glass dosimeters, normally used to measure gamma ray and electron fields, have been used to measure gamma ray and coexistent thermal neutron fields in the presence of a fast neutron field. The dosimeter is darkened in proportion to the energy deposited and this darkening is calibrated in known gamma ray and thermal neutron fields. A three-glass sandwich was used to distinguish the effects of gamma rays and thermal neutrons. The sensitivity of the glass to various energy gamma and x-rays is presented. A simple spectrophotometer is used to measure the transmission of light through the glass dosimeter.