The interactions between the cytotoxic paddlewheel dirhodium complex [Rh₂(μ-O₂CCH₃)₄] and the model protein bovine pancreatic ribonuclease (RNase A) were investigated by high-resolution mass spectrometry and X-ray crystallography. The results indicate that [Rh₂(μ-O₂CCH₃)₄] extensively reacts with RNase A. The metal compound binds the protein via coordination of the imidazole ring of a His side chain to one of its axial sites, while the dirhodium center and the acetato ligands remain unmodified. Data provide valuable information for the design of artificial dirhodium-containing metalloenzymes.