A new experimental and theoretical method is described to determine the temperature of a rich flame from its luminosity in a time-resolved way. It differs from existing methods in supplying the wavelength dependence of the flame emissivity <(epsilon)over cap>. The light emitted by the combustion is collected by a spectrograph and an image-intensifying streak-camera. Temperatures are derived from the spectral data. This pyrometric method is more precise and more meaningful than previous ones because it is free of any arbitrary assumptions on the wavelength dependence of <(epsilon)over cap>. The experimental scheme is relatively simple and very well suited for various applications. It was tested by investigating a candle light: a butane gas burner, and a series production diesel engine. It yielded temperatures with a total error of about 4%.