Poly(ethylethylenimine), PEEI, was prepared from poly(ethylenimine) by reductive alkylation with acetaldehyde. Samples of PEEI and poly(methylenimine), PMEI, complexed with LiCF3SO3 were prepared and characterized using differential scanning calorimetry and FT-IR. Small differences in the room temperature spectra of the two complexes were noted; these differences were due to the presence of a CH2 group in the side chain of PEEI. The predominant form of cation–anion interactions was a contact ion pair. As the samples were heated, a transition from ion pairs to “free” ions was observed, with most of the change occurring between 140 and 150°C in both PEEI and PMEI complexes. Thermal cycling established that the transition was irreversible in the time frame of the cycling experiments. Two-dimensional correlation spectroscopy did not show any significant intensity or frequency changes in bands sensitive to cation–polymer interactions during any heating or cooling cycle.