This paper presents the use of UV-vis and Raman spectroscopy to measure the state of a solid in a multiphase reactor. A slurry of particles and a packed bubble column were used. As this study is a proof of principle a model for an active catalyst system, insoluble pH indicators deposited on the particles and on a solid foam packing (used as the packing material in the packed bubble column) are used. This insoluble pH indicator has different UV-vis and Raman spectra according to the liquid pH. The experimental results indicate that for the slurry the spectra obtained from the moving particles can be used to characterize the state of the pH indicator. Bubbles in the packed bubble column result in disturbances in the UV-vis and Raman 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.