Abstract Purified natural rubber was irradiated in vacuo with a 4 MeV electron beam at dose rates of the order 10 6 –10 7 rad/min. The gas evolved comprised almost entirely hydrogen. The yield of carbon-carbon crosslinks was estimated from swelling data with corrections for the influence of the initial molecular weight of the rubber, chain fracture, and entanglements, as G (X) = 1·3. When irradiated at −196°C the rubber assumed an intense colour which rapidly disappeared when it was allowed to warm up. Under these conditions G (H 2 ) was little changed but the yield of crosslinks was apparently markedly reduced. The value of G (H 2 ) was not affected by irradiation at room temperature at relatively low dose rates (10 3 rad/min: 60 Co) but there was a limited amount of data indicating a higher G (X) value of about 1·6. The yield of hydrogen could be markedly reduced with large amounts of an additive such as carbon tetrachloride. However, increased yields were obtained with additives which contained a weak carbon-hydrogen bond, e.g. hydroquinone. These effects have been discussed in terms of competition by the additive for hydrogen atoms which otherwise undergo metathetical and addition reactions with isoprene units. The reduction of unsaturation on irradiation of a hexaisoprene, squalene, was found to be very much smaller than previously reported estimates for the polyisoprene rubber.