This paper describes a study on the surface modifications induced by the photodegradation process in polymeric materials. Several types of polyolefins (linear low density polyethylene, polypropylene) were u.v. degraded. SEPAP 12-24 apparatus (Service d’Etude du Photovieillissement Accelere des Polymeres) was used to accelerate the degradation process. Scanning electronic microscopy of photodegraded specimen surfaces revealed the unexpected formation of treeing figures, which disappear during the storage period. The phenomenon observed on the photooxidized samples has two plausible explanations based on the presence of photoproducts in u.v.-irradiated samples : the first is that the formation of treeing figures occurs in the acceleration device; the second is that a high field is required to facilitate their formation. Some staining procedures are used to better address the findings.