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 particle,; 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.