This study deals with the observed changes in the structure and performance of polyethersulfone (PES) membranes due to in situ formation and ex situ addition of silica particles (SiO2). Hydrolysis and condensation of tetraethyl orthosilicate (TEOS) inside the PES polymer matrix and the reaction of TEOS with ammonium hydroxide were chosen to form in situ and ex situ SiO2 formations, respectively. The resultant structure confirmed by X-ray diffraction for the composite PES membranes showed the retention of the amorphous nature even after the addition of SiO2. The FTIR study revealed the functional groups corresponding to silica networks with enhanced OH signatures on the surface of the composite membranes. Field emission scanning electron microscopic images showed the variation in the surface and cross-sectional structures for the pure and composite membranes. Considerable reduction in the thickness of the skin, difference in the pore structure and 'finger-like' cross-sectional morphology with the presence of SiO2 was observed in PES membranes. Both SiO2/PES composite membranes were showed a minor change in their glass transition temperature (T-g). The ex situ methodically formed composite membrane displayed an increase in the pure water flux and decrease in bovine serum albumin rejection as compared to in situ and pure PES membranes. These kinds of composite membranes can be utilized for water treatment applications demanding higher water flux.