A particulate form (hot particles) of the radioactive fallout in Poland after the accident at the Chernobyl Nuclear Plant has been studied. Parameters of the effective reactor campaign are obtained from the characteristics of -activity of the hot particles. The numerical INV program written to calculate time evolution of eight isotopes in the reactor is described. On the basis of these calculations the measured hot particles are classified into two groups: those originating from the first load and those from younger fuel rods. A procedure for dating younger hot particles is proposed. The results of the analyses constitute a basis for other investigations of hot particles.