Laboratory studies have been conducted to determine the rate and mechanism of methylglyoxal (MGLY, CH3COCHO) photolysis under tropospheric conditions. The UV/visible absorption across sections of MGLY as a function of wavelength have been measured at three different temperatures, 248 K, 273 K, and 298 K. The absorption cross sections show only small variations (≤10%) over this temperature range. The results at 298 K are in agreement with the values published by and differ by almost a factor of 2 from earlier work published by . The quantum yields and products of the photolysis are reported for various wavelength bands between 240 and 480 nm. The dominant pathway for photolysis at wavelengths relevant to the troposphere produces two radicals, CH3COCHO+h→CH3CO+HCO. The measured absorption cross sections and quantum yields are used to determine a methylglyoxal lifetime for photolysis of (2.7¿0.7) hours, for a solar zenith angle of 0--60¿. The kinetics and mechanism for reaction of HO2 with methylglyoxal were also determined. Although this reaction is unimportant for the atmospheric oxidation of methylglyoxal, the results were used in the interpretation of the quantum yield experiments. ¿ American Geophysical Union 1995