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.