The matrix isolation technique has been employed to isolate and characterize the initial intermediate in the reaction of (CH 3 ) 3 Al with O 2 . Using a flow gas reactor system immediately prior to matrix deposition, evidence of reaction was noted only when a constricted nozzle was employed. Under these conditions, four product bands that can be assigned to a single product species were formed. Oxygen isotopic labeling and comparison with literature spectra identified the product species as monomeric (CH 3 ) 2 AlOCH 3 . Experimental and theoretical evidence are also presented to support the conclusion that (CH 3 ) 3 AlOO is not the initial product of this reaction. Heating the deposition line to as high as 350 o C destroyed the initial bands and led to the formation of one or more secondary species. These could not be conclusively identified; the well-known dimer [(CH 3 ) 2 AlOCH 3 ] 2 is the most likely candidate. The fact that the reaction only occurred with a constricted nozzle suggests that O 2 is reacting with the trimethylaluminum dimer, across the four-membered ring to form two molecules of (CH 3 ) 2 AlOCH 3 .