Time evolution of the lamellar self-assembling behaviour of a commercial styrene-butadiene-styrene (SBS) star block copolymer, submitted to two different thermal treatments under thermooxidative degradation conditions, is reported. A microphase separation process, leading to partially oxidized butadiene and polystyrene-rich copolymer, occurs in the block copolymer exposed at 150 o C in air. The timing of the microphase separation depends upon previous thermal treatment, however, when the microphase separation takes place, the same evolution can be observed independently of the thermal treatment. The evolution of the morphologies has been observed by Tapping Mode Atomic Force Microscopy (TM-AFM). The evolution of the oxidation reactions and their influence on the molecular mass has been followed by thermogravimetric analysis (TGA) and gel permeation chromatography (GPC). Fourier transform infrared (FTIR) spectroscopy has been used to analyse the chemical changes involved in the nano/microphase separation process.