Polyvinylidene fluoride (PVDF) membranes are widely used in membrane industry, especially for membrane bioreactors (MBRs). Many PVDF membranes contain residual polyvinyl pyrrolidone (PVP) that acts as a pore-forming additive. The presence of residual PVP in some commercial PVDF membranes is often not specified and, therefore, its impact is mostly overlooked in the literature. In this study, we investigated the effect of PVP leaching on membrane structure and its implication on membrane fouling in a lab-scale MBR. PVP leaching can occur in two ways: (1) over the course of filtration from PVDF/PVP blend membranes or (2) prior filtration operation by treating the aforementioned membrane. We prepared PVDF, PVDF/PVP blend, and PVDF/PVP blend post-treated with sodium hypochlorite (NaClO), then assessed their performance. Leaching of PVP prior to the filtration operation significantly enlarged membrane pore size and thus reduced the membrane resistance. However, this advantage was dismissed during operation in MBR because PVP leaching also induced surface hydrophobicity that promoted membrane fouling, suggesting the detrimental effect of post-treatment. For PVDF/PVP blend sample, two counter-acting phenomena occurred: (1) slow leaching of PVP which gradually enlarged the membrane pores and rendered the membrane surface more hydrophobic and (2) adsorption of foulants that formed a secondary layer atop of membrane surface, restricted the pore mouth, and rendered the surface hydrophilic. The findings are significant since the change of membrane morphology over the course of filtration, as demonstrated in this study, is often overlooked when assessing membrane performance.