Four types of polyhedral oligosilsesquioxanes (POSS) were incorporated into chitosan by solution blending to fabricate composite membranes. The thermal stability of the composite membranes was improved compared with the neat chitosan membrane. For the mechanical properties, the stiffness of the composite membranes was weakened by the presence of POSS. However, the effects of POSS on the strength and elongation at break of the composite membranes depended on the type of POSS incorporated. Hydrophilic POSS increased the ductility of the composite membranes and maintained their strength, while hydrophobic POSS made the membranes more brittle and less rigid compared with the neat chitosan membranes. These significant differences were due to the different compatibility and interfacial adhesion between POSS and chitosan. The permeation of sodium chloride and riboflavin in the composite membranes was also influenced by the type and content of POSS in the membranes. Composite membranes containing hydrophilic POSS showed higher partition coefficients but lower diffusion coefficients of riboflavin, compared with the composite membranes containing hydrophobic POSS. This indicates that the transport of riboflavin is retarded by the interactions between riboflavin and the membrane. This study suggests that the structure and physical properties of chitosan membrane can be modified by the incorporation of a small amount of POSS molecules. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 427-435, 2011