The mechanism of energy separation in a free jet and its enhancement by acoustic excitation are investigated using an experimental technique to measure instantaneous velocity and total temperature simultaneously. The measured velocity and total temperature data are analyzed and compared by both time-averaged and spectral methods. By introducing the skewness of the total temperature fluctuation, the characteristics of energy separation can be identified. The results show that the frequencies of dominant total temperature fluctuation coincide with those of velocity fluctuation which represent the passing frequencies of vortices at give locations. This confirms that the mechanism of energy separation is induced by the motion of the coherent vortical structure which generates pressure fluctuation with the flow field. Spectral analysis of the data with acoustic excitation indicate that the enhancement of energy separation by acoustic excitation should result from vortex pairing processes induced by the acoustic excitation. (C) 2003 Elsevier Ltd. All rights reserved.