A chemical species in a turbulent flame may show changes in concentration brought about by chemical reaction or by physical mixing, and the ratio of the rates of these processes is the Damkohler number. * The additives sodium, indium, lead and acetylene generate charged species in the flame, and in a turbulent hydrogen diffusion flame of nozzle Reynolds number 10,000, the Damkohler numbers lie in the range 0 to 14, so that either physics or chemistry may be chosen as dominant. The instantaneous concentration of the charged species may readily be monitored by a Langmuir probe, and the autocorrelogram and power density spectrum of the fluctuating signal determined by suitable data processing. Consideration of the actual magnitude of the quantities involved shows that autocorrelogram is a good discriminator between physical and chemical dominance and depends markedly on the Damkohler number.