The barrier layer of a composite reverse osmosis (RO) membrane should ideally be very thin in order to obtain a high permeation flux. At the same time, it should also be mechanically strong to withstand the high pressures typically encountered in RO. This suggests that the mechanical strength of the barrier layer should bean important factor determining its performance. The rupture strength of interfacially polymerized polyamide thin films was measured directly by pendant drop mechanical analysis (PDMA). Permeation experiments were then per-formed to correlate the mechanical strength to the permeation performance of the composite membranes. The experiments indicate that the permeation behavior of composite membranes with a high rupture strength barrier layer agree well with the solution-diffusion transport mechanism. However, for low rupture strength barrier layers, the permeation behavior of the composite membrane deviates significantly from the solution-diffusion mechanism, and agrees well with the solution-diffusion imperfection model. We believe that this occurs due to a partial break-up of the barrier layer in the large pore region at the surface of the support layer; the break-up leads to the introduction of an additional hydrodynamic flow at high pressure drops. The results also indicate that the rupture strength information is very valuable in determining the interfacial polymerization (IP) conditions that lead to optimum membrane performance.