Solid [{Cu(MePY2)}(2)O-2](2+) is spectroscopically characterized using resonance Raman and X-ray absorption spectroscopy, for which the former technique probes the nature of the O-O bond and the latter defines the Cu-Cu interaction. In contrast to the crystal structure obtained for [{Cu(MePY2)}(2)O-2](2+), which shows an "intermediate" Cu2O2 core (Cu-Cu = 3.3 Angstrom and O-O = 1.6 Angstrom), resonance Raman peaks characteristic of both a side-on peroxide-bridged dicopper(II) core and bis-mu-oxo dicopper(III) core are observed. The bis-mu-oxo isomer is estimated to be present at approximately 5-20%. A good fit is obtained for EXAFS data for solid [{Cu(MePY2)}(2)O-2](2+) using an 80:20 ratio of Cu-Cu separations of 3.6 Angstrom (characteristic of a side-on peroxide-bridged copper core) and 2.8 Angstrom (associated with a bis-mu-oxo dicopper core). Analysis of the edge region places an upper limit on the amount of bis-mu-oxo isomer present in the solid at 40%. The factors governing the presence of bis-mu-oxo and/or side-on peroxide cores in solution for differing ligand systems are considered, and the contribution of the bite angle of the equatorial nitrogen atom donors is explored. The reactivity of [{Cu(MePY2)}(2)O-2](2+) in solution is correlated with the presence of the bis-mu-oxo core, using frontier molecular orbital theory.