A combination of thermogravimetric analysis and temperature programmed decomposition was employed to identify the outgassing species, the total amount of outgassing, and the outgassing kinetics of high density polyethylene (HDPE) in a vacuum environment. The isoconversional kinetic analysis was then used to analyze the outgassing kinetics and to predict the long-term outgassing of HDPE in vacuum applications at ambient temperature. H2O and CnHx, with n as high as 9 and x centering around 2n, are the major outgassing species from solid HDPE, but the quantities evolved can be significantly reduced by vacuum baking at 368 K for a few hours prior to device assembly.