The method of histograms is applied to the determination of polydispersity of particles and molecules in solution from fluorescence correlation spectroscopy (FCS) data, This is an ill-posed problem, which can be overcome by using a common strategy for imposed regularization and constraint conditions. The method developed far evaluating the polydispersity is tested on both computer-generated correlation curves and real FCS data. The results obtained show that FCS measurements can be successfully used for the determination of polydispersity of suspensions, with an efficiency comparable to that of photon correlation spectroscopy (PCS). The advantage of FCS, however, is its better sensitivity to small particles (size <50 nn) and molecules in dilute solutions, as well as its better selectivity, The usefulness of FCS for environmental chemistry is discussed with regard to the obtained results.