The structure and stability of the adduct formed in the reaction between Auoxo3, a dinuclear gold(III) compound, and the model protein hen egg white lysozyme (HEWL) are investigated by X-ray crystallography, UV-Vis absorption spectroscopy and circular dichroism (CD). It is found that Auoxo3 breaks down completely, undergoes reduction and produces reactive gold(I) species able to bind the protein and form stable derivatives. The behaviour of Auoxo3 is compared with that of two analogous gold(III) complexes previously studied: a few significant differences are highlighted. The general implications of these new results for the mode of action of cytotoxic gold complexes are discussed.