Many reactions can yield ketones on polyethylene oxidation. Separation of the experimental data into parts arising from contributions by different mechanisms is one of the main problems in attributing ketone formation to specific reactions. This is achieved in polyethylene melts by fitting separately the data pertaining to the initial and those relative to the advanced stages of processing to an equation formed by a linear and a quadratic term in processing time. It shows that ketone formation in the high temperature range (170–200°C) can be separated into two main parts that can be attributed to two different mechanisms. The first part corresponds to ketone formation without an induction period, i.e. at a constant rate at a given temperature. It is based on a cage reaction that is part of the chain propagation reaction. It involves a peroxy radical and the tertiary hydrogen atom of the secondary hydroperoxide whose formation on hydrogen abstraction by a peroxy radical is at the origin of the new peroxy radical. Monomolecular decomposition of the α-hydroperoxy alkyl radical formed in the reaction yields a ketone and a hydroxyl radical, which is continuing chain propagation. This part of ketone formation can be accounted for by solution-type homogeneous kinetics.The second part of the ketone formed shows a significant induction period which increases with decreasing temperature. It corresponds to ketone formation resulting from various hydroperoxide decomposition reactions. This mechanism, overall the same as that for alcohol formation, is well accounted for by the heterogeneous kinetics already used for explaining alcohol formation.