The photoluminescence (PL) spectrum of nanometer-sized particles of PbI2, at 77 K, is dominated by an acceptor-like exciton emission. When the specimens were irradiated with visible light at temperature of 70 degrees C, the exciton energy was blue-shifted and its intensity decreased. This blue shift is a manifestation of a quantum size effect, and it indicates the reduction of the particle’s size. The changes in the PL spectra showed strong dependence on the radiation conditions. These results suggest that the irradiation produces chemical decomposition of Pb-I bonds. Moreover, retreatment of the particles with iodine vapors nearly regenerated the original PbI2 particle’s size.