commented: I would like to make some comments concerning the interaction of acoustic waves with adsorption. For the two cases described below, the adsorption of species on the electrode of the microbalance was followed by another technique. First, adsorption of albumin can be simultaneously followed by the QCM and by the change of the double layer capacity of the electrode. Fig. 5 above shows the two plots simultaneously obtained after addition of 0.5 g l~1 of albumin. The time constants obtained through a Ðtting procedure with the sum of two exponentials are quite similar. Fig. 6 shows the time variation of the double layer capacity for an oscillating and a non-oscillating quartz crystal. It shows that the amplitude which indicates the amount of albumin adsorbed is larger for a non-oscillating quartz.1 Secondly, a quite di†erent process was studied by means of a QCM: electrochemical precipitation of calcium carbonate from hard waters. Fig. 7 shows the simultaneous plot of the mass of scale obtained from the QCM and the change of current due to the blocking of the active surface by the insulating scale (chronoamperometric curve). Fig. 8 shows chronoamperometric curves plotted on an oscillating (curve 2) and a nonoscillating (curve 1) quartz. Obviously the current decrease is faster for the nonoscillating quartz which demonstrates a faster scale precipitation.2 These two examples show that the vibration of the quartz crystal of the QCM decreases the adsorption rate and then experiments carried out on QCM have to be compared, at least quantitatively, with experiments carried out on a motionless electrode. 1 F. Lacour, R. Torresi, C. Gabrielli and A. Caprani, J. Electrochem. Soc., 1992, 139, 1619. 2 C. Gabrielli, M. Keddam, A. Khalil, G. Maurin, H. Perrot, R. Rosset and M. Zidoune, J. Electrochem. Soc., to be published.