Many microfiltration (MF) and ultrafiltration (UF) membranes are subjected to repetitive cycles of chemical cleaning as a fouling control strategy. The exposure to aggressive chemical cleaning agents could result in membrane degradation, leading to compromised process performances. Little is known on the nascent changes occurring at the molecular and structural level of a membrane in progressive ageing over prolonged periods. Accelerated ageing studies were conducted on polyvinylidene fluoride (PVDF) and polypropylene (PP) hollow fibre and flat sheet membranes with sodium hypochlorite (NaOCl). The suitability of different membrane characterisation techniques (based on their reliability and sensitivity) as predictive tools for membrane ageing depends on membrane configuration and materials. In a second series of experiments, fouling/cleaning cyclical runs were conducted under controlled environment to assess the relative contribution of chemical cleaning in the overall performances of the porous membranes. Results revealed the multiple degradation steps caused by the chemical agent to the overall changes of the membrane characteristics. They include: (1) removal of preservative compounds during initial wetting, (2) gradual degradation of the additives mixed with the core membrane material during the manufacturing process, along with some apparent opening of the membrane pores, (3) complete destruction of additives, resulting in decrease of membrane performances, (4) small degradation of the core membrane material leading to loss of integrity.