The stability of graphene oxide (GO) membranes in a sodium sulfate aqueous solution is reported. The mechanism for the stability was interpreted with the assistance of molecular dynamic (MD) simulations as follows: GO disperses uniformly in water because it is hydrated. However, the hydration of GO is weakened when Na 2 SO 4 is present because the Na + and SO 4 2− ions are preferentially hydrated over GO. Thus the movement of GO in the Na 2 SO 4 solution becomes restricted and it is difficult for the GO in a GO membrane to uniformly disperse in a Na 2 SO 4 solution even with ultrasonic treatment.