The reaction of trimethylaluminum with the hexameric tert-butylalumoxane, [( t Bu)Al-(µ 3 -O)] 6 , has been investigated. Reaction of [( t Bu)Al(µ 3 -O)] 6 with 1 equiv of AlMe 3 results in the formation of two isomers (A and B) of the hybrid tert-butylmethylalumoxane, [Al 7 (µ 3 -O) 6 -( t Bu) 6 Me 3 ]. The structures of compounds A and B, as determined by NMR spectroscopy and mass spectrometry, consist of [Al 6 (µ 3 -O) 6 ( t Bu) 5 Me] alumoxane cages, formed via tert-butyl/ methyl exchange, in which one of the edges of the Al 6 O 6 cage is complexed to the ( t Bu)-AlMe 2 formed during alkyl exchange. The difference between the isomers results from the geometric relationship of the cage Al-Me group and the opened edge. The activity of [Al 7 -(µ 3 -O) 6 ( t Bu) 6 Me 3 ], for the [Me 2 C(Cp)(Flu)]ZrBz 2 -catalyzed polymerization of 1,5-hexadiene, is significantly increased in comparison to [( t Bu)Al(µ 3 -O)] 6 . The effect of additional equivalents of AlMe 3 on the cocatalytic activity of [Al 7 (µ 3 -O) 6 ( t Bu) 6 Me 3 ] suggests that a maximum activity is obtained at a [( t Bu)Al(µ 3 -O)] 6 to AlMe 3 ratio of 1:6. Under conditions of equal Al:Zr ratio the [( t Bu)Al(µ 3 -O)] 6 (AlMe 3 ) 6 system has a higher activity than a representative sample of commercial methylalumoxane (MAO). 1 H NMR suggests that the reaction of [( t Bu)Al-(µ 3 -O)] 6 with 6 equiv of AlMe 3 yields ( t Bu)AlMe 2 as the only tert-butyl-containing species and a proposed AlMe 3 "free" form of MAO. Whereas the activity of [Al 7 (µ 3 -O) 6 ( t Bu) 6 Me 3 ] and MAO shows slight inhibition by the addition of Al( i Bu) 3 , the activity of the [( t Bu)Al(µ 3 -O)] 6 / (AlMe 3 ) 6 system is unaffected. The reaction of Cp 2 Zr(CD 3 ) 2 with [Al 7 (µ 3 -O) 6 ( t Bu) 6 Me 3 ] demonstrates that methyl exchange does not occur between a metallocene and the alkyls of the alumoxane cage, but does occur with the complexed ( t Bu)AlMe 2 .