Establishing how polymeric vectors such as polyethylenimine (PEI) bind and package their nucleic acid cargo is vital toward developing more efficacious and cost-effective gene therapies. To develop a molecular-level picture of DNA binding, we examined how the Raman spectra of PEIs report on their local chemical environment. We find that the intense Raman bands located in the 1400-1500 cm-1 region derive from vibrations with significant CH2 scissoring and NH bending character. The Raman bands that derive from these vibrations show profound intensity changes that depend on both the local dielectric environment and hydrogen bonding interactions with the secondary amine groups on the polymer. We use these bands as spectroscopic markers to assess the binding between low molecular weight PEIs and single-stranded DNA (ssDNA). Analysis of the Raman spectra suggest that PEI primarily binds via electrostatic interactions to the phosphate backbone, which induces the condensation of the ssDNA. We additionally confirm this finding by conducting molecular dynamics simulations. We expect that the spectral correlations determined here will enable future studies to investigate important gene delivery activities, including how PEI interacts with cellular membranes to facilitate cargo internalization into cells.