The adsorption of porphyrin molecules out of ethanol solution onto TiO2 particles of nano-porous TiO2 films was studied by means of optical absorption (UV-Vis) spectroscopy. The process was found to be limited by aggregation of dye molecules. From the temperature dependence of the process, an activation energy of 0.18 +/- 0.02 eV was deduced. It is correlated to the interaction energy of pi-stacked porphyrin molecules. Aggregation-breaking additives change the up- take kinetics in a way that it becomes similar to the kinetics of non-aggregating molecules. (C) 2008 Elsevier B.V. All rights reserved.