The chemical degradation processes of HDPE at 110 o C in air have been studied using light microscopy, FTIR analysis, micro-hardness and OsO4 staining. It is shown that carbonyl groups C=O, (as detected by FTIR) are formed only in the surface layer and the effect on mechanical properties is marginal. On the other hand the formation of isolated and conjugated C=C double bonds (as detected by OsO4 staining), show that cutting properties and micro-hardness are significantly altered. The progress of the degraded layer follows a diffusion process with sharp front and saturated concentration behind. An attempt is made to explain these observations by formation and mobility of free radicals. It is also shown that processing method, injection or compression moulding, can significantly influence the degradation process and the mechanical performance of the moulded component. based on initiation, propagation and termination of radicals, resulting in formation of more stable structures. Most reactions are dependent on the presence of oxygen and extraction of hydrogen. Solubility of oxygen in HDPE is sufficient for reactions, even at room temperature. Major functional groups formed are alkyl, alkoxy or peroxy radicals. Hydroperoxide are also formed and can decompose to hydroxy and peroxy radicals. Tertiary carbons are more reactive than secondary carbons and therefore have a shorter lifetime. Conjugated carbons are more stable than carbonyls and thus it can be expected that they dominate. Cross-linking also occurs and it will increase the strength but also the brittleness of the polymer. The number of possible reactions is very large and it is necessary to select those which are most common and result in more stable structures. The degradation process is most often studied using FTIR spectroscopy considering the formation of carbonyls (C=O) as the major chemical reaction. However the FTIR analysis indicate that most carbonyls are formed within the surface layer, exponentially decreasing with depth, leaving the major part of the polymer thickness un-degraded. This can be explained by photo-oxidation (17, 18). In the present work oxidation of HDPE is studied by comparing FTIR results with changes in mechanical properties, such as micro-indentation and cutting properties which, together with osmium tetroxide staining, reveal the concentration and progress of C=C double bonds. Injection moulded samples as well as compression moulded samples, (which have more homogeneous structure) are used. This shows the importance of polymer processing methods on the performance and durability of polymer components.