In contrast to the electronically excited propynal at 193 nm undergoing aldehyde C–H and C–C bond ruptures, on pulsed TEA-CO₂ laser irradiation, multiphoton vibrationally excited propynal undergoes concerted dissociation generating CO and acetylene. Vibrational excitation in the CO product is detected immediately following the CO₂ laser pulse by observing infrared (IR) emission at 4.7 μm. The decay of the IR emission was studied as a function of propynal pressure. A vibrational–vibrational relaxation rate constant of CO (ν ≥ 1) by propynal is found to be 1540 ± 200 Torr⁻¹ s⁻¹. With the collisionless dissociation of propynal, the evaluated unimolecular rate constant of (1.5 ± 0.2) × 10⁷ s⁻¹, vis-a-vis RRKM calculations, gives an average IR multiphoton excitation level of propynal as 75 ± 4 kcal mol⁻¹.