The Q branch of the ν 3 stretching fundamental of 32 SF 6 (ca. 947.6–948.3 cm− 1) has been recorded at 109–142 K with a resolution of< 10− 4 cm− 1, and the individual transitions have been assigned. Several series of lines arising from different subbranches can be followed up to high J values (J≈ 80 in one case). Some of these subbranches form prominent band-heads that dominate the gross structure of the Q branch, especially at higher temperatures. A moderately strong transition at 947.742 cm− 1, displaced− 8 MHz from the P (16) CO 2 laser line, has been assigned as Q (38) F 1 0+ E 0+ F 2 0; 39 other transitions that fall within±300 MHz of the laser line have also been identified. The Q branch of 33 SF 6, present in natural abundance (0.76%), has been recored and analyzed; the isotope shift is 8.97 cm− 1.