The structure of the novel cadmium phenanthroline thiosulfate poly[[(1,10-phenanthroline-κ²N,N′)cadmium(II)]-μ₃-thiosulfato-κ³S:S:O], [Cd(S₂O₃)(C₁₂H₈N₂)]_n, with a pentacoordinated Cd centre, is reported. It forms linear chains built up around a 2₁ axis and is isostructural with the known bipyridine homologue. The structure is also compared with a previously reported closely related monoaqua monohydrated phase, where the Cd²⁺ cation is hexacoordinated. The incidence of weak C–H⋯O interactions in the determination of its general packing properties is discussed.