A detailed investigation of a valence tautomeric (VT) transition for the new complex [Co-III(3,5-DBCat)(3,5-DBSQ)(py)(2)]/[Co-II(3,5-DBSQ)(2)(py)(2)] (1) is reported, where 3,5-DBCatH(2) is 3,5-di-tert-butyl-catechol, 3,5-DBSQH is 3,5-di-tert-butyl-semiquinone and py is pyridine. Complex 1 exists as a mixture of the two valence tautomers, with the relative proportion of each depending on the external conditions. Three differently solvated forms of the complex have been synthesized and variable temperature structural and magnetic investigations of one of these, 1 center dot 0.5py, reveals that this compound undergoes a thermally-induced VT transition from the [Co-III(3,5-DBCat)(3,5-DBSQ)(py)(2)] tautomer at temperatures below 150 K to a 1 : 1 mixture of the two tautomers at temperatures above 300 K. The VT transition may also be photo-induced at 9 K, affording a similar mixture of the two tautomers. In both cases the incomplete transition is attributed to the presence of pi-pi stacking interactions between the pyridine molecules of solvation and one of the two crystallographically independent complex molecules, which inhibits the expansion of this molecule that would accompany a VT transition. Studies on alternatively solvated forms 1 center dot 2MeCN and 1 center dot 1.67hexane also suggest a significant dependence of the VT transition on solvation-induced packing effects and/or intermolecular interactions.