The Criegee intermediates, carbonyl oxides produced in ozonolysis of unsaturated hydrocarbons, play important roles in atmospheric chemistry. The two conformers of CH 3 CHOO exhibit distinct reactivity toward several atmospheric species, but a distinct conformer-specific probe is challenging because ultraviolet and infrared absorption bands of syn- and anti-CH 3 CHOO overlap at low-resolution. Employing a quantum-cascade laser and a Herriott cell, we recorded the O-O stretching bands of CH 2 OO and syn-CH 3 CHOO in region 880-932 cm -1 at resolution 0.0015 cm -1 . In addition to completely resolved vibration-rotational lines of CH 2 OO extending over 50 cm -1 , some spectral lines associated with hot bands were identified. Spectral lines solely due to syn-CH 3 CHOO were also identified. Probing these lines, we determined the rate coefficient for the self-reaction of syn-CH 3 CHOO to be k self = (1.6 ± 0.6 0.5 ) × 10 -10 cm 3 molecule -1 s -1 , about twice that of CH 2 OO.