Linear poly(N-methylethylenimine), LPMEI, a methylated derivative of linear poly(ethylenimine), 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',N"-pentamethyldiethylenetriamine, PMDETA, containing either dissolved LiCF3SO3 (LiTf) or NaCF3SO3 (NaTf). During these studies, crystalline compounds were discovered and characterized by differential scanning calorimetry, Fourier transform infrared spectroscopy (IR), and Raman spectroscopy. Crystals of the NaTf complex, (PMDETA)NaTf, were of sufficient size to allow a structure determination by X-ray diffraction. The (PMDETA)NaTf crystallizes as two dimers with different symmetries in a monoclinic unit cell in the P2(1)/c space group. A spectroscopic comparison of PMDETA, the crystalline complex PMDETA-LiTf, crystalline (PMDETA)NaTf, and their corresponding salt solutions over a composition range of 1.7:1 to 6.7:1 (PMDETA:MTf molar ratio, M = Li+, Na+) was carried out with IR and Raman spectroscopy. The dominant species in the (PMDETA),NaTf solutions is the triple cation [Na2Tf](+), even at a 6.7:1 PMDETA:NaTf molar composition.