Equilibrium calculations were carried out on a computer for a system containing tungsten, bromine, oxygen, hydrogen, and carbon. The computer program receives as input the standard enthalpy and entropy of formation and the heat capacity as a function of the temperature for each species considered in the calculations. For initial fill and operating conditions compatible with those in actual tungsten bromine lamps, the concentrations of all components in any phase considered were calculated in the temperature range 500°–3500°K at 500° intervals under conditions of chemical equilibrium. The validity of these calculations on the lamp performance is based on the assumption of negligible departure from localized equilibrium and on the accuracy of the thermodynamic values used.