The characterization of a polyclonal antibody produced via immunization with an [Os(bpy)2dcbpy] hapten is described. Bpy is 2,2′-bipyridine and dcbpy is 2,2′-bipyridine-4,4′-dicarboxylic acid. The cross-reactivity of the antibody for the Ru(II) analogue of the hapten was also investigated. Large increases in the emission and luminescent lifetime of a series of Os and Ru complexes were observed on binding of the antibody. Association equilibrium constants were derived from luminescence titration data and were found to be 5.6 × 108 and 5.0 × 108 M−1 for [Os(bpy)2dcbpy] and [Ru(bpy)2dcbpy], respectively. Spectroscopic changes were likely due to the exclusion of H2O from the complex/antibody binding cleft and blocking of vibrational relaxation pathways of the Os/Ru excited state. D2O/H2O experiments confirmed that the antibody protected approx. 82% of [Os(bpy)2dcbpy] and 80% of [Ru(bpy)2dcbpy] from excited state deactivation by the aqueous solvent.