Understanding the removal of trace organic contaminants is critical for membrane applications in water recycling. This study investigates the relationship between trace contaminant sorption and their rejection by nanofiltration (NF) membranes. A mass balance is developed that quantitatively links the rejection decline over time seen with some sorbing compounds to the total mass found sorbed on the membrane. The sorbed mass of perfluorooctane sulfonamide (FOSA) and fluoxetine evaluated from the mass balance agreed to within approximately 30% of the quantity analytically determined via extraction. Static sorption experiments show that sorption takes place predominantly within the polyamide separating layer of the membrane. Finally, the relationship between the steady-state rejection and sorption tendency of ten trace organic compounds is elucidated. A greater tendency to sorb results in lower steady-state rejection, both when comparing compounds of similar size, as well as when comparing the same compound under different conditions. As a result, a major finding is that in the presence of competitive sorption, that is, the presence of other trace organic compounds in the membrane matrix, some compounds sorb less and are therefore rejected more than when these compounds are alone in the feed. At no point during experimentation was any effect on the water flux observed.