A compact photon-correlation spectrometer for basic and applied research in physics, chemistry, biology, medicine, engineering, and environmental technology as well as for educational laboratory courses in these subjects has been developed. The instrumental setup enables one to make absolute measurements of the sizes of particles suspended in liquids in the range from 0.001 to 5 μm. The measurements are fast, lasting usually from seconds to several minutes. Real-time size monitoring, such as of kinetic aggregation processes, is also possible. The optical arrangement of the spectrometer even makes it possible to measure light scattering in opaque systems which are characterized by strong light absorption. The quantity of the sample to be studied can be quite small, starting from 0.01 cm3. The system includes specially designed software for performing data interpretation and for implementing fitting procedures, for exchange of data with other programs, and for automation of measurements of long duration or of sequences of measurements. Measurements verifying the accuracy of the system are presented for latex suspensions, for aniline dye dissolved in water, and for dilute solutions of polystyrene in toluene.