The coordination structure in the solid state and solution complexation behavior of 6-(tetrazol-5-yl)-2,2'-bipyridine (HN(4)bipy) with samarium(III) was investigated as a model system for actinide(III)/lanthanide(III) separations. Two different solid 1:2 complexes, [Sm(N(4)bipy)(2)(OH)(H2O)(2)] (1) and [Sm(N(4)bipy)(2)(HCOO)(H2O)(2)] (2), were obtained from the reaction of samarium(III) nitrate with HN(4)bipy in isopropyl alcohol, resuspension in N,N-dimethylformamide (DMF), and slow crystallization. The formate anion coordinated to samarium in 2 is formed by decomposition of DMF to formic acid and dimethylamine. Time-resolved laser fluorescence spectroscopy (TRLFS) studies were performed with curium(III) and europium(III) by using HN(4)bipy as the ligand. Curium(III) is observed to form 1:2 and 1:3 complexes with increasing HN(4)bipy concentration; for europium(III), formation of 1:1 and 1:3 complexes is observed. Although the solid-state samarium complexes were europium(III) solution complex in ethanol was not identified with TRLFS. The determined conditional stability constant for the 1:3 fully coordinated curium(III) complex species is more than 2 orders of magnitude higher than that for europium(III) (log beta(3)[Cm(N(4)bipy)(3)] = 13.8 and log beta(3)[Eu(N(4)bipy)(3)] = 11.1). The presence of added 2-bromodecanoic acid as a lipophilic anion source reduces the stability constant for formation of the 1:2 and 1:3 curium(III) complexes, but no ternary complexes were observed. The stability constants for the 1:3 metal ion N(4)bipy complexes equate to a theoretical separation factor, SFCm/EuIIIIII approximate to 500. However, the low solubility of the HN(4)bipy ligand in nonpolar solvents typically used in actinide lanthanide liquid liquid extractions prevents its use as a partitioning extractant until a more lipophilic HN(4)bipy-type ligand is developed. confirmed as 1:2 species the 1:2