Photon correlation spectroscopy (PCS) is a technique to measure rapidly particle size in the sub-micrometre region. The use of PCS is, however, limited by concentration. The upper limit is due to multiple scattering of the incident light and the lower limit is determined by the fact that fluctuations of the number of particles in the measuring zone have a significant influence on the appal ent diffusion coefficient. In this paper a signal processing method is described which differentiates this influence. With this system the lower limit is no longer limited to about 100 particles in the measuring volume corresponding to a concentration of 10(9) particles/cm(3). The limitation is now the intensity of the scattered light, which becomes too weak at a concentration of about 50 particles/cm(3). As a consequence of this work, a revision to the basic theory of PCS may be necessary. Moreover, the new processing method also permits the measurement of the particle concentration in the sample.