Long-chain linear alkanes have been used as model compounds for polyethylene in an attempt to identify the chemical nature of crosslinks formed in polyethylene when it undergoes gamma-irradiation in the presence of acetylene. IR and UV spectral analysis of alkanes and polyethylene following acetylene-sensitized irradiation shows the formation of vinyl, trans-vinylene, and diene groups. A correlation of the conditions of formation suggests that in polyethylene the vinyl groups are restricted to amorphous regions, diene groups are restricted to the crystalline regions, and trans-vinylene groups are formed in both regions. There is no information on the nature of crosslinks. C-13-NMR analysis of alkanes following irradiation of molten alkanes in the presence of C-13-enriched acetylene has shown that a range of saturated alphatic structures are formed by inclusion of acetylene molecules in the alkane structure. They include ethyl branches, gamma-branches, -CH(CH3)-, and -CH2-CH2- branches as the major species; the latter two are potential crosslink sites in the irradiation of polyethylene. In addition, the NMR analysis confirmed that the C atoms of the vinyl groups come from acetylene molecules and those of the trans-vinylene groups come from alkane molecules. Data on irradiation of the alkanes in the crystalline state showed that acetylene inclusion in the alkane structure is minimal under these conditions. The principal finding of this work is that acetylene can be incorporated as saturated aliphatic crosslinks in the amorphous regions of polyethylene during high-energy irradiation.