Pressure retarded osmosis has the potential to produce renewable energy from natural salinity gradients, like that which occurs when the ocean meets a fresh water river.  The performance of this system depends largely on the semi-permeable membrane used to separate the two bodies of water.  The pores of the membrane must be large enough to allow small molecules, like water, to pass through but not so large that salt molecules can pass.  Molecular simulations indicate that carbon nanotube membranes are about 10,000 times more efficient than commercially available membranes.  Here, graphene and carbon nanotube membranes on polytetrafluroethylene (PTFE) filter supports were tested.  After 120 hours, graphene sheets continued to demonstrate osmosis, demonstrating that graphene membranes are extremely stable.  Both graphene and carbon nanotube (CNT) membranes exhibited increases in pressure greater than 1,000 Pascals, dependent on the membrane material and salt concentration used.  The measurable hydrostatic pressure created by graphene membranes indicates that graphene could be effectively used for salinity power generation, however further development is necessary in order to optimize membrane performance.