The structural properties of zinc, cadmium, and mercury metallothioneins (MT) have been studied at 77 K using X-ray absorption fine structure spectroscopy (XAFS). The nearest neighbor bond lengths and coordination numbers for Zn(7)MT 2 and Cd-7-MT 2 are in close agreement with the previously reported values. Strikingly different structural parameters have been obtained for the mercury-containing metallothioneins. For Hg-7-MT 2 the nearest neighbor Hg-S bond length and coordination number are determined to be 2.33 +/- 0.02 Angstrom and 2, respectively. A new structure is proposed for Hg-7-MT 2 formed by addition of Hg2+ to Zn-7-MT at pH 7 in which each Hg(Il) is coordinated by four thiolate sulfur but with two unusually short bonds and two unusually long bonds which exhibit a much larger disorder than the shorter bonds. For the Hg-18-MT 2 species formed by adding Hg2+ to apo-MT 2 at pH 2, the local structure is shown to involve nearest neighbor Hg-S bond lengths of 2.42 +/- 0.026 Angstrom with 2-fold sulfur coordination plus an Hg-Cl shell at 2.57 +/- 0.03 Angstrom with an apparent coordination number 0.61 and large disorder.