Linear poly(N-methylethylenimine) (LPMEI), a methylated derivative of linear poly(ethylenimine) (LPEI), shows potential as a polymer electrolyte host. The interactions of LPMEI with lithium and sodium cations are modeled by solutions of N,N,N',N'-tetramethylethylenediamine, TMEDA, containing either dissolved lithium triflate (LiTf) or sodium triflate (NaTf). During these studies, crystalline compounds were discovered and characterized by X-ray diffraction, differential scanning calotimetry, and Fourier transform infrared spectroscopy (FT-IR). The TMEDA:LiTf crystallizes as dimers in an orthorhombic unit cell in the Pccn space group; however, the TMEDA:NaTf crystallizes as tetramers in a triclinic unit cell in the P (1) over bar space group. A spectroscopic comparison of TMEDA, TMEDA:LiTf crystal, TMEDA:NaTf crystal, and their corresponding salt solutions over a composition range of 5:1 to 20:1 (nitrogen: cation, molar ratio) is carried out using FTIR and Raman spectroscopy. The triple cation species [Li2Tf](+) is the dominant species in the TMEDA:LiTf solution and exclusively present in the crystal. However, the dominant species in the TMEDA:NaTf samples is the aggregate [Na3Tf](2+). The N-C-C-N dihedral angle, which appears to be a mixture of gauche minus and trans in both LiTf and NaTf solutions, changes to gauche minus upon crystallization. Finally, the vibrations of individual TMEDA molecules appear to be completely decoupled in the TMEDA:LiTf crystal, although each dimeric unit contains two TMEDA molecules.