We have studied the ageing effects on the photoluminescence (PL) of porous silicon (PS) prepared by anodisation in HF solution of low concentration (3% ). It was found that the PL behaviour within oxidation is different from that elaborated in habitual condition. The natural oxidation of PS layers shows two opposite behaviours of the PL before stabilisation. The PL intensity decreases significantly after an initial strong and fast increase. At low temperature, the PL exhibits large band and multi-peaks structures. It was shown by electron paramagnetic resonance (EPR) and Raman measurements that a possible phase transition from crystalline to amorphous phase can occur. We show that each transformation is related to preparation conditions.