The reaction of the cytotoxic compound dirhodium tetraacetate with a B-DNA double helical dodecamer was studied by X-ray crystallography and mass spectrometry. The structure of the dirhodium/DNA adduct reveals a dimetallic center binding to an adenine via axial coordination. Complementary information has been gained through ESI MS measurements. Comparison between the present data and those previously obtained for cisplatin indicates that the two metallodrugs react with this DNA dodecamer in a significantly different fashion.