A Prussian Blue colorimetric method has been developed to determine the number of conjugated polyenes present in commercial elastomers. These groups disappear at the onset of oxidation of unvulcanized lithium-poly (isoprene) : their content decreases by about 50 per cent during the first hour heating at 100° C in air. Simultaneously, a great number of volatile oxidation products are formed since the early stage of aging. The major products, the concentrations of which increase throughout heating until oxidation is advanced, are presumably produced by random attack on the regular isoprene units. The present investigation has made it possible to isolate from this group of compounds 4-methyl-4-vinyl-butyrolactone and 4-hydroxy-2-butanone, previously unrecognized, in addition to levulinaldehyde, methacrolein, and methyl vinyl ketone previously identified by Bevilacqua. An attempt was made to explain their origin. In addition to the compounds just mentioned, minor products corresponding essentially to unsaturated aldehydes and esters were also obtained. They might arise from the decomposition of conjugated polyenes which disappear at the start of oxidation. They have not yet been collected in sufficient amounts to confirm their structures and more experimental work is required to establish the mechanism by which they are formed.