Laser photolysis of UF6 gas was performed in a supersonic nozzle reactor for molecular laser isotope separation. The nascent product UF5 formed UF5 nanoparticles in the reactor. Comparison of the experimentally observed average particle diameter with that predicted by the Brownian coagulation model presented by Lee et al. (1990, Aerosol Sci. Technol. 12, 457-462) suggests that nanoparticle growth proceeded under the condition of a particle sticking probability close to unity and no significant gas mixing in the reactor.