BACKGROUND In this work, a series of graphene oxide-polyethersulfone (GO-PES) composite flat sheet (FS) and hollow fiber (HF) membranes were fabricated by blending 0.5 and 1.0 wt% of GO into the PES matrix and utilized for oil-water separation. GO was first prepared and characterized by using Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) before being used as membrane fillers. RESULTS Interestingly, although similar dope composition was used for the fabrication of HF and FS membranes, their morphology and pore size varied significantly owing to the way the membranes were fabricated. Significant enhancement of hydrophilicity was observed for both composite FS and HF membranes, where the water contact angle value decreased from 74.8 degrees to 42.2 degrees and 71.4 degrees to 49.8 degrees, respectively. The flux of the composite membranes increased up to 150% of the bare membranes especially when 1.0 wt% of GO was added. Although higher oil rejection (99%) was observed for HF membranes that possess smaller pores, the permeation flux was maintained due to the improved flow dynamic. Lower oil rejection (up to 50%) was observed for FS membranes, which might be due to its big pore sizes particularly at the bottom surface. As more oil droplets formed on the membrane surface and prevented water molecules to pass through the membrane, fouling might occur rapidly. CONCLUSIONS The results obtained in this work suggest that surface hydrophilicity, pore size of membranes and oil-water separation performances was greatly affected by membrane shape. Owing to many advantages of HF membranes, this type of membrane has great potential for commercial applications. (c) 2020 Society of Chemical Industry