The thermal-induced and photoinduced valence tautomerism of a series of Co(dioxolene)(2)(4-X-py)(2) complexes (dioxolene = 3,5-di-tert-butylcatecholate or 3,5-di-tert-butylsemiquinonate; 4-X-py = 4-(X)pyridine, X = H (1), OMe (2), Me (3), CN (4), Br (5), NO2 (6)) is described. The thermal valence tautomerism (Is-Co-III(SQ)(Cat)(4-X-py)(2) <-> hs-Co-II(SQ)(SQ)(4-X-py)(2)) is only observed for complexes 4, 5, and 6 where each is accompanied by a hysteresis loop of ca. 5 K. When a crystalline sample of 4-6 is held at 10 K in a SQUID magnetometer and irradiated with white light (lambda = 400-850 nm), the hs-Co-II tautomer is formed. When the light source is removed, and the sample is slowly heated, the hs-Co-II tautomer persists until ca. 90 K, approximately 40 K higher than the thermal stability of previously reported complexes. Heating and cooling the sample while maintaining irradiation results in the appearance of a new light-induced thermal hysteresis loop below 90 K (Delta T = ca. 25 K). Below 50 K, the hs-Co-II tautomer displays temperature-independent relaxation to the Is-Co-III form, and above 50 K, the relaxation is thermally activated with an activation energy E-a > ca. 1500 cm(-1). The coordination geometry (trans-pyridines), pyridine substitution, and crystal packing forces conspire to create the comparatively thermally stable photogenerated hs-Co-II tautomer, thus providing an excellent handle for molecular and crystal engineering studies.