The ligand N,N'-diethyl-2,3-dihydroxyterephthalamide (ETAM) has been characterized as a chelator for Zr(IV), Ce(IV), and Th(IV). The K+ salts of the complexes [Zr(ETAM)(4)](4-), [Ce(ETAM)(4)](4-), and [Th(ETAM)(4)](4-) were prepared in a MeOH solution containing H(2)ETAM, the corresponding M(acac)(4), and 4 equiv of KOH. Single-crystal X-ray diffraction analyses are reported for K-4[Zr(ETAM)(4)] (C2/c, Z = 8, a = 27.576(3) Angstrom, b = 29.345(3) Angstrom, c = 15.266(2) Angstrom, alpha = 90degrees, beta = 118.688(4)degrees, gamma = 90degrees), [Me3BnN](4)[Th(ETAM)(4)] (P (1) over bar, Z = 2, a = 13.7570(3) Angstrom, b = 13.9293(3) Angstrom, c = 26.9124(6) Angstrom, alpha = 99.941(1)degrees, beta = 94.972(1)degrees, gamma = 103.160(1)degrees), and the dimeric (NMe4)(4)[Th(ETAM)(3)MeOH](2) (P2(1)/c, Z = 4, a = 18.2603(9) Angstrom, b = 18.5002(9) Angstrom, c = 19.675(1) Angstrom, beta = 117.298(1)degrees). Solution thermodynamic studies were used to determine formation constants (log K-1 and esd) for Th(IV)-ETAM log K-110 =17.47(1), log K-120 = 13.23(1), log K-130 = 8.28(3), log K-140 = 6.57(6), and log beta(140) = 45.54(5). These results support the hypothesis that the terephthalamides are high-affinity chelators for the actinide(IV) ions and thus promising ligands for use in nuclear waste remediation.