Pressure-Retarded Osmosis (PRO) is a method for converting salinity gradients to energy. In this process, a hydro-static counter-pressure is applied to retard osmotic flow through a semi-permeable membrane and gain mechanical energy. The maximum power output is known to occur at counter-pressures that are about one half of osmotic pressure in the draw solution. In this communication we obtain an explicit criterion for the optimal hydrostatic counter-pressure and use it for a parametric analysis of its dependence on such system parameters as the properties of membrane active and support layers and solute concentrations in the draw and feed solutions. In particular, we show that besides the draw concentration this pressure noticeably depends on the feed concentration. This finding is important for the design of PRO modules where this concentration can considerably change along the module. In addition, we demonstrate that the optimal counter-pressure can be noticeably reduced by decreasing the S-parameter, which is important for the design of optimal PRO membranes. (C) 2014 Elsevier B.V. All rights reserved