Abstract: Infrared spectra of SF_6 trapped in cryogenic matrix of argon have been measured. Five peaks are observed at the frequencies of 938.51, 938.05, 937.55, 939.82 and 936.22 cm~(-1) for the v_3 mode. Varying the concentration of SF_6 in argon matrix and diffusion studies indicate that the 937.55 cm~(-1) peak is due to SF_6 aggregate and the others arise from monomer. The changes in relative intensities with annealing show that there are at least three different trapping sites for monomeric SF_6 in argon lattice. The result that the increase in intensity of the peak at 938.03 cm~(-1) parallels the decrease in that of the peak at 938.51 cm~(-1) implies a temperature-induced-change in the site population.