Photon Density Wave (PDW) spectroscopy[1] is one of the very few optical methods for the characterization of the optical properties of highly concentrated liquid emulsions or suspensions. Such materials often exhibit multiple light scattering - the basis for successful PDW experiments. Since this analytical technique can quantitatively separate light absorption and light scattering properties[2], chemical and structural information of such materials can be obtained. PDW spectroscopy has been applied to investigate the Ostwald ripening of concentrated o/w-nano-scaled emulsions, produced by a phase inversion temperature (PIT) emulsification. Even for volume fractions of 20% of the oil phase (isohexadecane), ripening rates in dependency of the emulsifier concentration (tetraoxyethylene laurylether, 2-8 vol%) can be obtained with high precision and high temporal resolution. Furthermore, the inline monitoring of high solid content (up to 55 wt%) polymer particles has been investigated. Polyvinyl acetate nanoparticles (400-700 nm) were successfully synthesized by starved feed radical emulsion polymerization, characterized by PDW spectroscopy, and compared to reference methods like DLS. As a third topic, hard sphere and electrostatic nano-particle interactions have been studied by PDW spectroscopy. Through appropriate theoretical modelling, the experimentally obtained optical properties of polymer particles in highly concentrated suspensions (> 5 vol%) can be used to determine their ionic strength, particle surface charge and size. Conversely, PDW spectroscopy can be applied to adjust the optical properties of suspensions with nano-scaled particles, e.g. by changing the ionic strength of the system. [1] L. Bressel, R. Hass, O. Reich, J. Quant. Spec. Rad. Transfer 2013, 126, 122–129. [2] O. Reich, H.-G. Löhmannsröben, F. Schael, Phys. Chem. Chem. Phys. 2003, 5, 5182-5187.