Thin-film-composite reverse-osmosis (RO) membranes were prepared by the interfacial polymerization of trifunctional 1,3,5-benzentricarbonyl chloride (TMC) with difunctional 1,3-benzendiamine (MPDA) or 1,4-benzendiamine (PPDA). The meta-positioned polyamide (MPDA/ TMC) resulted in higher water flux but lower salt rejection than the para-positioned polyamide (PPDA/TMC). To understand this behavior, we studied various factors including the thickness, rupture strength, chemical properties, and solubilities of the water and salt in the thin-film polyamide. The thin films made from MPDA and PPDA possessed similar thicknesses and rupture strengths adequate for with standing the RO operation pressure. However, the meta-positioned polyamide had higher hydrophilicity and greater molecular chain mobility than the para-positioned polyamide, resulting in higher water flux. In contrast, the para-positioned polyamide had lower salt solubility and lower molecular chain mobility than the meta-positioned polyamide, thereby possessing higher salt rejection.