Polymerization and chemical reactions taking place in supercritical fluids (SCFs) are often combined with the formation and precipitation of insoluble products. The properties and quality of a formed product are significantly influenced by its particle size. To control and regulate the product precipitation during an SCF process measurement, techniques are required to monitor online particle sizes and particle growth. This paper presents the adaptation of the so-called three-wavelength-extinction technique to a dynamic sc-CO2 process containing fine solid particles or droplets of immiscible liquids. This technique allows the simultaneous determination of mean particle sizes and particle concentrations in dispersed systems. The measuring principle is based on the extinction of monochromatic light at three different wavelengths due to absorption and dispersion, as described by the MIE theory. However, for the application of the three-wavelength-extinction technique to a SCF process, the strong influence of pressure and temperature on the optical properties of supercritical fluids must, in particular, be considered. Systematic studies investigating TiO2 particles suspended in sc-CO2 have shown that reliable particle size measurements can be achieved whenever the specific refractive index of sc-CO2 is taken into account. Particle sizes determined by the three-wavelength-extinction technique under sub- and supercritical conditions were in excellent accordance with particle size's obtained by conventional offline measurement techniques. In addition to the characterization of fine particles, the three-wavelength-extinction technique is also suitable for size measurements of droplets in sc-CO2. For example, droplets being formed during the mixing of dimethylformamide (DMF) and sc-CO2 and dimethyl sulfoxide (DMSO) could be observed and analyzed in real time. In general, one can conclude that the three-wavelength-extinction technique is a suitable tool for monitoring and analyzing the formation of particles and droplets during a supercritical fluid process.