This paper presents the use of ultraviolet-visible spectroscopy (UV-Vis) spectroscopy in a slurry of particles, a packed bubble column, and a trickle bed to assess the changes in the state of an active component on the surface of the solid support. As a model system, insoluble pH indicators deposited on the particles and on a solid foam packing (used as the packing material in the packed bubble column and the trickle bed) are used which fluoresce different UV-Vis spectra according to the liquid pH. The experimental results indicate that for the slurry the UV-Vis spectra obtained from the moving particles can be used to characterize the state of the pH indicator and to determine the transition point. The UV-Vis spectra can also be used to characterize the concentration of particles. Bubbles in the packed bubble column result in disturbances in the UV-Vis spectra collected from the pH indicator adsorbed to the solid surface and this interference is removed successfully with a newly developed tolerance-and-averaging method. In the trickle bed the liquid film flowing over the solid surface does not disturb the UV-Vis spectra. An abrupt change in the state of the pH indicator is therefore observed successfully. (C) 2009 Elsevier Ltd. All rights reserved.