We have demonstrated through experimental and theoretical study that membrane sublayer structure has significant influence on the performance in forward osmosis (FO) processes. Cellulose acetate (CA) hollow fiber membranes with different sublayer structures have been fabricated by varying the bore fluid composition during dry-jet wet spinning and then characterized in terms of pore structure of the selective layer, porosity of the sublayer, pure water permeability (PWP) coefficient, salt permeability coefficient and salt rejection. The water flux and reverse salt flux of these membranes are evaluated in the pressure-retarded osmosis (PRO) mode and FO mode. It is observed that varying the sublayer structure does not have apparent influence on the FO performance under the PRO mode but has significant influence on the performance under the FO mode. The different performance under different operation modes is resulted from more severe concentration polarization (CP) in the FO mode. The characteristics of the membrane sublayer structure are the origin of CP phenomenon in the FO mode. Through NF tests and FO tests with low draw solution concentrations on one of the membranes, the reflection coefficients (sigma(0)) are determined as 0.77 and 0.99 for NaCl and MgCl(2) draw solutes, respectively. A low reflection coefficient indicates a low effective driving force in FO processes resulted from serious leakage of draw solutes from the draw solution to the feed. Thus, the reflection coefficient cannot be assumed to be equal to 1 and its significance must be considered when modeling FO performance using NaCl as the draw solute. The theoretical study reveals that the sublayer porosity, tortuosity and thickness have great impact on membrane performance when the draw solution flows against the sublayer. The desired FO membrane should have no sublayer. Since the phase inversed membrane may always have a sublayer, the preferred sublayer has high porosity, low tortuosity and small thickness. However, FO membranes for water reuse may have less structure requirements than those for seawater desalination. (C) 2011 Elsevier B.V. All rights reserved.