The advantages of using acetylene gas as a sensitizing agent in the irradiation crosslinking of polyolefins is now well documented.1-3 Improvements in mechanical properties can be achieved with about 10% of the dose required for irradiation in an inert atmosphere, when acetylene gas is present.3 Post-irradiation treatment with acetylene is also effective.4 The chemistry associated with the process is, however, much less well understood. In fact, in comparison with the reaction of alkenes with free radicals, the reaction of alkynes with free radicals is a very neglected area of chemistry. This is partly due to the low degree of specificity observed in these reactions. In the reaction of acetylene with irradiated polyolefins, it is known that very little acetylene is used in the process and that high gel contents are achieved at relatively low doses. Consequently, identification of the chemical groups formed is hampered by the requirements of high sensitivity and solubility.