The clustering of UF6 seeded in Ar was observed in a continuous supersonic Laval nozzle flow. The onset conditions for UF6 clustering were investigated by measuring the FTIR spectra of UF6 monomer and clusters in the nozzle. The onset conditions for the clustering, temperature, density of UF6 (or partial pressure), and cooling rate, were determined as {130 K, 0.009 mol m(-3) (10 Pa), 0.6 x 10(6) K s(-1)} and {155 K, 0.06 mol m(-3) (80 Pa), 1.5 x 10(6) K s(-1)}. The onset temperature determined here was higher by 40-50 K than that determined by a light-scattering method. The frequency shift of the main peak of the UF6 clusters from the monomer peak was about -17 cm(-1), which was smaller than the shift of the crystalline UF6 by about 11 cm(-1). The increase in temperature caused by the heat of condensation and the change of the spectra of UF6 clusters with the growth after the onset were also observed. It was shown that the clustering rate due to the collision between the monomer and cluster is much higher than that due to the collision between the monomers.