Two compound classes have been identified from the reaction of trivalent lanthanide nitrates with tetramethylsuccinamide (TMSA). The nature of these Ln-TMSA compounds has been examined in the solid phase by thermogravimetric analysis (TGA) and single-crystal X-ray diffraction and in solution by infrared spectroscopy. Isostructural, 10-coordinate compounds with a 2:1 TMSA:Ln stoichiometry were found for the lanthanides La and Ce. Single-crystal X-ray diffraction analysis of the Ln-TMSA nitrate compounds, Ln = La and Ce, reveals a polymeric structure with each metal center containing one chelating bidentate succinamide and two bridging bidentate succinamides. For the remaining lanthanides Pr, Nd, Gd, Yb, and Lu, a series of isostructural, 9-coordinate compounds containing a 3:2 TMSA:Ln ratio were formed. Single-crystal X-ray diffraction analysis of the Ln-TMSA products, Ln = Ce, Pr, Nd, Gd, and Yb, reveals these compounds to be dimeric, with each metal center containing one chelating bidentate succinamide and two bridging bidentate succinamides. The solution stoichiometries were examined in acetonitrile-water mixtures and support the stoichiometries observed in the solid-state structures. Cerium nitrate appears to be a special (and transitional) lanthanide in its reaction with TMSA. Although infrared and TGA analyses indicate a 2:1 ligand-to-metal product, crystals for both the 2:1 and 3:2 compounds were isolated. No evidence for such multiple forms was found with any of the other lanthanides.