Reduced graphene oxide (rGO) has huge potential for membrane applications owing to its appropriate interlayer spacing (0.34-0.37 nm) that enables it to block salt ions as small as Na+ with high precision. However, fabrication of uniform rGO membranes is a great challenge because of the loss of its polar functional groups during preparation from graphene oxide (GO). Although there have been several studies on GO membranes and a few on rGO for water purification, very few attempts to understand the role of oxygen-containing functional groups in successful rGO membrane formation. The present work deals with the investigation of the key factors and functional groups that govern membrane formation. This work also utilizes the facile approach of synthesizing reduced GO by environmentally viable hydrothermal reduction. Further characterizations show that the hydroxyl and carboxyl groups are principally responsible for the formation of uniform rGO membranes. The rGO (treated at 160 degrees C for 2 h) membrane with a small amount of unreduced GO presented shows the lowest Na+/Cl- ion permeation with the highest membrane flux, which is suggested to be a potential candidate for water desalination.