The temperature- and pressure-driven solid state valence tautomeric transformations of [Co-II(3,5DTBSQ)(2)(phen)]. C6H5-CH) and nonsolvated [Co-II(3,5DTBSQ)(2)(phen)] have been studied by EXAFS and XANES. When the toluene solvate sample is cooled it converts in the 270-215 K region (T-c = 240 K) from a high-spin Co-II complex at higher-temperatues to a low-spin Co-III complex at temperatures below similar to 215 K. EXAFS data indicate that the HS-Co-II tautomer has bond lengths of Co-O = 2.08 +/- 0.02 Angstrom and Co-N = 2.13 +/- 0.02 Angstrom, whereas the LS-Co-III tautomeric complex is smaller with Co-O = 1.91 +/- 0.02 Angstrom and Co-N = 1.93 +/- 0.02 Angstrom. The XANES spectra obtained in the 270-215 K interconversion region were simulated to obtain a plot of the fraction of HS-Co-II complexes, n(HS-Co-II), vs temperature. This plot of n vs T agrees with that obtained from magnetic susceptibility data for the toluene solvate. In the XANES spectral region transitions characteristic of either a HS-Co-II or a LS-Co-III complex could also be used to monitor the valence tautomeric conversion. The nonsolvated phen complex does not convert at low temperatures (2 K) to a LS-Co-III tautomer, However, both complexes could be driven from the larger HS-Co-II form to the smaller LS-Co-III form upon application of pressure. EXAFS and XANES data readily detected these changes. For the toluene solvate, this conversion occurred within the 0.075-0.700 GPa range with a midpoint of P-c up arrow = 0.370 GPa. The nonsolvated complex required higher pressures of 0.10-2.5 GPa with P-c up arrow = 1.1 GPa, These are the first reported examples of valence tautomeric transformations driven by pressure. Both the thermally driven and pressure-driven valence tautomeric transformations are found to be reversible.