Two model linear polyethylenes of different molecular weights and very narrow molecular weight distributions were subject to gamma-ray irradiation under vacuum. Irradiations were conducted at two temperatures: 25 and 135 degrees C. Number average and weight average molecular weight of irradiated samples was measured by LALLS and membrane osmometry, and the distribution of molecular weights was studied by GPC. The presence of a low but measurable amount of scission was verified. A mathematical model of the irradiation process was developed, which gave very good predictions of the measured values.