Intermolecular energy transfer rate from a photo-excited Malachite Green (MG) molecule to solvent molecules was investigated by the acoustic peak delay method of the transient grating technique and the transient absorption method. It is found that the rate significantly depends on the concentration of MG; the peak delay time decreases with increasing the concentration. This unexpected concentration dependence is explained in terms of the ionic character of this molecule. (C) 2002 Elsevier Science B.V. All rights reserved.