The enthalpies of combustion and formation of two samples of linear polyethylene which differ only in the degree of crystallinity have been determined in an oxygen bomb calorimeter. For the two samples the degree of crystallinity, the enthalpy of combustion at 298.15 K, and the enthalpy of formation at 298.15 K, were respectively: 72 percent, -651.16 ± 0.12 kJ · mol-1, -28.18 ± 0.13 kJ · mol-1 for the less crystalline sample; and 96 percent, -650.27 ± 0.12 kJ · mol-1 and -29.08 ± 0.12 kJ · mol-1 for the more crystalline sample. The values are per mole of CH2. Uncertainties listed are estimates of accuracy of approximate 95 percent confidence limits. The results of previous determinations by other investigators are discussed briefly.