The photolysis of methylglyoxal (CH3COCHO) in the presence of synthetic air was studied by laboratory experiments in a static reactor in order to determine its atmospheric Lifetime. Quantum yields of the molecular photolysis products CO and HCHO were determined at 298 K as a function of wavelength (260 less than or equal to lambda less than or equal to 440 nm) and pressure (30 less than or equal to P less than or equal to 900 Torr) using an optical resolution of 8.5 nm. The results can be distinguished with respect to both UV/VIS-absorption bands of methylglyoxal. For the short-wavelength band (260 less than or equal to lambda 320 nm) photolysis quantum yields were found to be unity, independent of wavelength and pressure, consistent with a dissociation mechanism yielding peroxy radicals and CO according to CH3COCHO --> (hv) -->(O)(M)(2,) CH3COO2 + HO2 + CO (P1). For the long-wavelength band (380 less than or equal to lambda less than or equal to 440 nn) two different processes were distinguished. The major process is photodissociation (P1) with quantum yields (phi D) decreasing with increasing wavelength and pressure following the Stern-Volmer relationship : 1/phi D(lambda) = 1/phi o(lambda) + P/k＇D(lambda) with phi o(lambda) = (8.15 +/- 0.5)(10-9) [exp(7131 +/- 267)] nm/gamma and k＇D(gamma) = (7.34 +/- 0.1)(10-9 Torr) [exp(8793 +/- 300)] nm/lambda. The minor process could be described by an I-I-atom transfer between electronically excited MG and ground-state MG, yielding the experimentally observed products [CH3COCHO]similar to + CH3COCHO o2,M CH3COO2 + HCHO + CO + CH3COO2(R10). The atmospheric lifetime due to photolysis (tau phot) was calculated using an atmospheric radiation model and the above expression, where phi o(lambda) = 1 for /Z ( 380 nm, resulting in tau phot = (4.1 +/- 0.7) h for a solar zenith angle of 50 degrees at ground level. Therefore, photolysis can be identified as the most important degradation process of atmospheric methylglyoxal.