Time-resolved photoluminescence measurements on size-selected silicon nanocrystals have been carried out in order to elucidate the nonexponential behavior of the photoluminescence decay kinetics. The nanoparticles are gas-phase synthesized, extracted as a supersonic beam, size selected, and deposited downstream as films of variable densities. The nanoparticle number densities were determined by atomic force microscopy. The photoluminescence properties appear totally independent of the film density. Even in the very low density film where nanoparticles are completely isolated from each other, the decay kinetics corresponds to a stretched exponential law. This means that the stretched exponential kinetics does not originate from the interaction between nanoparticles, but is actually a characteristic of the silicon nanocrystals.