The aqueous reaction of sodium pyrithione, (Na)mpo, with 243AmCl3·nH2O yields a dimerized complex, [243Am(mpo)2(μ-O-mpo)(H2O)]2·3H2O. This compound is compared with isostructural lanthanide pyrithionates, where dimerization across the 4f-block is observed to be dependent upon the size of the cation. Unlike in most reported Am(III) UV-visible absorption spectra, [243Am(mpo)2(μ-O-mpo)(H2O)]2·3H2O shows significant splitting in the fingerprint excitations. This is attributed to a unique ligand-field environment, where the Am-mpo bonds possess different bonding compared to the Nd(III) analog because of increasing covalent interactions.