A Raman-assisted crystallographic study on the adduct between AziRu, a Ru(III) complex with high antiproliferative activity, and RNase A is presented. The protein structure is not perturbed significantly by the Ru label. The metal coordinates to ND atoms of His105 or of His119 imidazole rings, losing all of its original ligands but retaining octahedral, although distorted, coordination geometry. The AziRu binding inactivates the enzyme, suggesting that its antitumor action can be exerted by a mechanism of competitive inhibition.