The first structurally characterized isobutyl-containing aluminoxane compound is presented. The (Al10O6Bu16)-Bu-i(mu-H)(2) (I) cluster is produced from neat octakis-isobutyltetraluminoxane ((Al4O2Bu8)-Bu-i) at 80 degrees C in 6-8 h followed by slow crystallization. The crystal is triclinic (space group P(1) over bar) with the molecule lying on an inversion center. This aluminoxane contains both nearly linear, 154(2)degrees, aluminum-bridging hydrides and three-coordinate aluminum sites. Solid-state Al-27 magic-angle spinning (MAS) NMR experiments were done at 19.6 and 40 T (833 MHz and 1.703 GHz, H-1) and at 30-35 kHz spinning speeds, leading to the determination of the C-q and eta values for the two four-coordinate Al sites and a lower limit of C-q for the three-coordinate Al site. Geometry-optimized restricted Hartree-Fock calculations at the double-zeta level of an idealized structure (methyl substituted, D-2h geometry) yielded C-q and eta in close agreement with experiment; C-q agrees within 3 MHz.