Vibrating in a Crowd High-vacuum molecular beam studies can probe the roles of specific vibrations and rotations on molecular reactivity with remarkably fine resolution. Glowacki et al. (p. 1066 ; see the Perspective by Tyndall ) now show, through a combination of spectroscopy and theoretical modeling, that oxidation of acetylene under effectively atmospheric conditions proceeds in part through vibrationally excited intermediates prior to collisional randomization.