Thin-film composite reverse osmosis (RO) membranes have been studied for nearly fifty years, gradually evolving to the high water flux, high salt rejection (typically >98%) materials used today. However, the high throughput and selectivity that make RO membranes viable candidates for desalination also make measuring their properties difficult. This study was undertaken to identify some important variables responsible for measured performance values (water flux and salt rejection) of commercial RO membranes. Crossflow filtration experiments were performed to characterize the water flux and NaCl rejection of three commercial polyamide reverse osmosis (RO) membranes (LE and XLE from Dow FilmTec and AG from GE Water and Process Technologies). Thorough cleaning of the crossflow system, along with following the recommended pretreatment and test conditions (including pressure, flowrate, temperature, feed pH, and feed filtration) of the manufacturer resulted in measured performance values in agreement with manufacturer benchmarks. Correction for the effect of concentration polarization also proved important. The influence of feed pH and continuous feed filtration on water flux and salt rejection was also determined. While rejection was strongly affected by feed pH, water flux was unaffected. Continuous filtration of the feed led to higher water flux and lower salt rejection than was measured in experiments with unfiltered feed, suggesting fouling of the membrane surfaces by the unfiltered feed. The flux and rejection of these three membranes obeyed a general tradeoff relation: membranes that exhibited higher flux had lower rejection and vice versa.