Paraffins and polyethylene can be crosslinked by exposure to ultra-violet radiation in the presence of sensitizers. Measurements were made of quantum efficiency, sensitizer concentration and degree of crosslinking, and compared with previous results for specimens exposed to $\gamma$ or electron radiation. It is found that crosslinking and degradation both occur with ultra-violet radiation in approximately the same ratio as for $\gamma$-radiation, thereby indicating that the same radical process is involved. The number of crosslinks is directly related to the number of sensitizer molecules destroyed, whereas only a fraction of the ultra-violet quanta absorbed result in any change in the sensitizer. Thus the overall reaction occurs in two distinct stages: (a) absorption of ultra-violet radiation by sensitizer, with a small probability of radical formation in the latter, (b) reaction between sensitizer radical and polymer molecule, this process being highly efficient. Once the radical has been formed on the sensitizer, the reaction proceeds in a manner almost parallel to that for $\gamma$-irradiation. There is no evidence of a distinctive reaction due to the ionization produced with the latter, high-energy, form of radiation. The influence of oxygen, molecular weight and physical state of paraffin on the reaction have also been investigated and the fate of the sensitizer determined. This technique offers an interesting possibility of studying some of the effects of ionizing radiation, by introducing an intermediate step which can be determined independently and quantitatively. Discrepancies between the effects of ultra-violet and $\gamma$-radiation would appear if ions play an important role in the reaction.