Permeate water quality from reverse osmosis (RO) and nanofiltration (NF) systems is a function of solvent and solute transport through the membrane film, to the permeate side. Mass transport theories describe two potential methods of solute transport through a membrane film: diffusion through molecular pores, via Brownian motion, at a rate dependent on the differential concentration; and, size-exclusion, in which solute passage is solely a function of a pore-size (all species smaller than the pore-size pass through, all larger species are completely rejected). Inorganic rejection, such as for dissolved ions, has been shown to be diffusion-controlled (permeate concentration increases as flux decreases and/or as recovery increases). However, data on organic compounds are less common and the method of transport is not as clear. This research investigated TOC, UV-254, color, SDS-THM, SDS-HAA, MIB, and geosmin transport in two NF systems to determine if passage was diffusion- or size-exclusion-controlled. A primary goal was to determine if flux and recovery influenced permeate quality to such a degree that water quality goals and regulatory requirements could only be met within a specific range of flux and recovery. This would be more likely if mass transport were diffusion-controlled as permeate concentration would vary with flux and recovery. Includes 4 references, tables, figures.