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Two gadolinium polyoxometalates, K9GdW10O36 and K11[Gd(PW11O39)2], have been evaluated both in vivo and in vitro as candidates for tissue-specific MRI contrast agents. T1-relaxivities of 6.89 mM−1 · s−1 for K9GdW10O36 and 5.27 mM−1 · s−1 for K11[Gd(PW11O39)2] are slightly higher than that of the commercial MRI contrast agent (Gd-DTPA). Both compounds bind with bovine serum albumin and human serum transferrin and favorable liver-specific contrast enhancement in in vivo MRI with Sprague-Dawley rats after i.v. administration has been demonstrated. Imaging studies demonstrate that the two agents have a long residence time, showing MR signal enhancement in the liver for more than 40 min, longer than commercially available contrast agents. In vivo and in vitro assays showed that GdW10 and Gd(PW11)2 are promising liver-specific MRI contrast agents and GdW10 may be used in the diagnosis of the pathological state. However, with the higher acute toxicity, the two gadolinium polyoxometalates need to be modified and studied further before clinical use. [Copyright &y& Elsevier]