The thermal- and photoinduced valence tautomerism of a cobalt bis(dioxolene) complex is described. The thermal conversion is precipitous, complete within 10K, and is accompanied by a 5K hysteresis loop (107 K < T-1/2 < 112 K). Rapid thermal quenching (300 K -> 10 K in ca. 5s) and photoinduced valence tautomerism result in trapping of the metastable Coll-state at low temperatures through intermolecular hydrogen bonding. This lattice stabilization results in unmatched kinetic and thermal stability for a valence tautomer from 10-50 K, with residual hs-Co-II persisting until about 90 K.