An algorithm has been developed to reconstruct the internal temperature distribution within a rising buoyant thermal from measurements of the radiances by an infrared imaging camera. The technique is applied to detonation of a stoichiometric mixture of methane and oxygen contained in a 3-m-diam mylar balloon. The temperature-time histories of the evolving fireball showed torus formation, followed by rapid cooling from entrained ambient air. Comparison of the temperature distribution with results of an Eulerian hydrodynamic computer code shows good agreement except at the center of the fireball. A possible explanation might be the lack of an entrainment mechanism in the hydrocode.