An experimental and theoretical investigation of pressure retarded osmosis (PRO) performance was conducted. The characteristic parameters of a commercial membrane were determined experimentally. Gross power density of up to 7.1 W/m2 was obtained, which is among the highest reported in the literature for a commercial membrane. The effect of operating conditions on membrane performance was also investigated. Results show that under typical test conditions, concentration polarization and spatial variations are minimized, but the resulting pressure losses due to friction lead to net power densities that are negative. Test conditions that favor net power density, and hence that are more appropriate for energy conversion applications, were identified. Under these conditions net power density of up to 4.5 W/m2 was observed.