A new mu-eta(2):eta(2)-peroxo dicopper(II) complex, [Cu-2(II)(alpha SP)(2)(mu-eta(2):eta(2)-O-2) (Bz(.))]SbF6 (alpha Sp = alpha-isosparteine, Bz, = benzoate) was synthesized by oxygenation of [Cu-1(alpha Sp)(CH3CN)]SbF6 with Bz(.) at -80 degrees C in acetone. X-ray crystallographic analysis of the dark blue crystal revealed that the "bridged butterfly core" is formed by bridging Bz(.) bound in the axial position. The weaker sigma-electron donation of aSp causes the axial coordination of Bz(.) to stabilize the butterfly-type mu-eta(2):eta(2)-peroxo dicopper(II) species as an intermediate of stepwise O-2-activation to bis(mu-oxo) dicopper(III) species.