Crosslinked polyethylene (XLPE) is a key material used for power cable insulation due to its electrical properties and structure. Here, it was shown that the technique of broadband dielectric spectroscopy can be used to analyze both by extracting the temperature coefficient of capacitance (TCC), dielectric loss, and AC conductivity. TCC for low-density polyethylene (LDPE) can be directly compared to its linear thermal expansion coefficient and used as a tool to compare different types of polyethylene. Degassing, a thermal treatment to remove volatile species and increase crystallinity, can be used to control TCC magnitude. In general, changes in the TCC magnitude are reflective of changes in the crystallinity and byproduct concentrations. Degassing also reduced the dielectric loss for both due to morphological changes and byproduct removal. However, it only reduces conductivity at elevated temperatures such as 90 °C for XLPE. Other processing parameters, as well as excessive byproduct concentrations, were also seen to influence TCC, dielectric loss, and conductivity.