Photoactivation of titanium oxide Degussa P25 in ethanol-m ethanol suspensions, with different molar ratios, was monitored in situ using a piezoelectric sensor. The process was studied in a specially designed cell in which the sample can be illuminated by a continuous UV Hg lamp to induce photoactivation. The evolution of the process was followed by sending a modulated red laser beam onto the sample positioned on top of a piezoelectric detector. The output voltage of the sensor is also modulated and its amplitude allows to follow the photoactivation process. In order to control the photoactivation process, the cell was fed with N-2 regulated flux. It is shown that when the sample is illuminated with a UV lamp, the amplitude of the piezoelectric signal falls gradually up to a value that remains constant. The amplitude of the photoactivation, the initial time and the velocity of photoactivation signal of titania are related to ethanol-methanol proportion. The mechanism associated with photoactivation and the role of ethanol-methanol proportion are discussed. (C) 2007 Elsevier B.V. All rights reserved.