The field of electrical insulation has improved considerably after the introduction of the first synthetic polymeric dielectrics. However, these materials are often considered the "weakest link" as they tend to degrade and eventually fail, thus compromising the reliability of devices and equipment. Such degradation is related to the occurrence of partial discharges (PDs), a phenomenon that is difficult to measure in actual applications. In the present work, the chemical evolution of the gas phase composition and polyethylene degradation in a polyethylene microcavity subject to a PD were studied using a thermodynamic model. Thermodynamic parameters for the depolymerization of polyethylene into ethylene were obtained using density functional theory calculations. The system evolution is studied minimizing the Gibbs free energy subject to the constraints of energy conservation explicitly considering the polyethylene surface among the reacting species. The model results can be interpreted in terms of the density of energy introduced by the partial discharge. At low discharge energy densities methane and carbon monoxide are the most abundant species produced by the discharge. As the energy density increases the model predicts that ethylene is formed in the gas phase because of the depolymerization of the polymer. At high energy densities carbon may be formed at the surface. In these conditions, the most abundant gas phase species predicted by the model are methane, carbon monoxide, hydrogen, water, and carbon dioxide. This is in good agreement with experimental observations reported in the polyethylene PD literature. In addition, the model is able to correlate the energy of the partial discharge to the increase of volume of the microcavity, thus providing a tool that may be used to predict polymer degradation due to PDs dissipated power. (C) 2018 Elsevier Ltd. All rights reserved.