Tunneling magnetoresistance (TMR), dynamic resistance and bias dependence measurements were performed on Co/Al2O3/Co magnetic tunnel junctions with a thin Cu layer inserted at either the Co/Al2O3 ("bottom") or Al2O3/Co ("top") interfaces. Careful comparative analysis allows detailed growth characteristics to be elucidated, as well as providing information on the underlying mechanisms behind spin polarized transport in these structures. Conductance for top dusted junctions is indicative of parallel Co/Al2O3/Co and Co/Al2O3/Cu junctions, consistent with three-dimensional growth of Co and Cu on Al2O3, while conductance for bottom dusted junctions show novel behavior dissimilar to either type of junction. The bias dependence of the TMR, surprisingly, is unaffected by either type of dusting. ©2000 American Institute of Physics.