We report the discovery of an eta(2)-SO2 linkage photoisomer in the osmium pentaammine coordination complex, [Os(NH3)(5)(SO2)][Os(NH3)(5)(HSO3)]Cl-4 (1). Its dark- and light-induced crystal structures are determined via synchrotron X-ray crystallography, at 100 K, where the photoinduced state is metastable in a single crystal that has been stimulated by 505 nm light for 2.5 h. The SO2 photoisomer in the [Os(NH3)(5)(SO2)](2+) cation contrasts starkly with the photoinactivity of the HSO3 ligand in its companion [Os(NH3)(5)(HSO3)](+) cation within the crystallographic asymmetric unit of this single crystal. Panchromatic optical absorption characteristics of this single crystal are revealed in both dark- and light-induced states, using concerted absorption spectroscopy and optical microscopy. Its absorption halves across most of its visible spectrum, upon exposure to 505 nm light. The SO2 ligand seems to be responsible for this photoinduced bleaching effect, judging from a comparison of the dark- and light-induced crystal structures of 1. The SO2 photoisomerism is found to be thermally reversible, and so 1 presents a rare example of an osmium-based solid-state optical switch. Such switching in an osmium complex is significant because bottom-row transition metals stand to offer linkage photoisomerism with the greatest photoconversion levels and thermal stability. The demonstration of eta(2)-SO2 bonding in this complex also represents a fundamental contribution to osmium coordination chemistry.