The effects of solvent composition, counterion, and temperature on the equilibrium between the isomeric complexes [((LCu)-Cu-iPr3)(2)(mu-eta(2):eta(2)-O-2)]X-2 [1(X)(2)] and [((LCu)-Cu-iPr3)(2)(mu-O)(2)]X-2 [2(X)(2); L-iPr3 = 1,4,7-triisopropyl-1,4,7-triazacyclononane; X = PF6-, ClO4-, or SbF6-], which differ with respect to the presence and absence, respectively, of an O-O bond, were examined via solution conductivity and W-vis and resonance Raman spectroscopic measurements. While previous work (Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Kaderli, S.; Young, V. G., Jr.; Que, L., Jr.; Zuberbuhler, A. D.; Tolman, W. B. Science 1996, 271, 1397-1400) had shown that 2(X)(2) (X = PF6- or ClO4-) formed exclusively in THF solution at concentrations >1 mM, new experimental results show that preferential aggregation/precipitation of 2(X)(2) occurs under these conditions. By using lower concentrations and/or X = SbF6-, apparently homogeneous solutions of mixtures of 1(X)(2) and 2(X)(2) in equilibrium in THF and THF/CH2Cl2 mixtures are generated. Evaluation of the temperature dependence of the equilibrium in THF allowed estimation of thermodynamic parameters [Delta H = 0.9(2) kcal mol(-1) and Delta S = 6(1) cal mol(-1) K-1] that are consistent with the small energetic differences between analogous isomers previously calculated using theoretical methods. Tn addition, by avoiding the selective aggregation/precipitation of 2(X)(2), the 1(X)(2)/2(X)(2) equilibrium was shown to exhibit a smooth dependence on the THF:CH2Cl2 ratio in solvent mixtures. The observed gradual shifts in the proportion of the equilibrating isomers as a function of solvent and temperature show that subtle environmental factors influence the equilibrium in similarly subtle ways, with implications for the potential involvement of both types of cores in biological and other catalytic processes.