A description is given of an instrument to determine the temperatures of rapidly varying flames. This instrument uses a photoelectric modification of the line‐reversal method, wherein the light beam from a temperature‐variable incandescent source is split into two monochromatic beams by means of mirrors and filters to impinge upon 931A multiplier phototubes. The phototube signals are sent to a mu‐bridge null detector, whose output is fed to an oscilloscope for photography of results. The instrument can detect 25°K temperature differences between about 1540° and 3100°K and three millisecond time differences. Typical time‐temperature results are given, along with the analytical procedures used.