Metastable sulfur dioxide linkage photo-isomers can be generated in a family of ruthenium tetraammine-based complexes in their single-crystal form; this imparts them with attractive prospects as holographic data storage media (dark state = 0; photo-isomer state = 1). Embedding these optically encoded single crystals into thin-film technology via a polymer host should present one possible option for their ultimate device application. Crystals of trans-[Ru(SO2)(NH3)4(H2O)][p-TolSO3]2([Ru]-SO2) were incorporated into a polyvinyl alcohol (PVA) matrix, by in situ synthesis of [Ru]-SO2 from the reaction of trans-[Ru(SO2)(NH3)4Cl]Cl and p-tolunesulfonic acid (p-TA) in the PVA matrix, and an associated in situ precipitating crystallisation. Transmission electron microscopy and diffraction on the resulting drop-cast thin films identified these [Ru]-SO2 precipitates and their crystallinity; low-temperature solid-state UV-vis …