Reverse osmosis membrane performance was evaluated using ASTM D 4516 and a modified form of the mass transfer coefficient (MTC) as described in the homogenous solution diffusion model (HSDM) on a common data set. Standardized salt passage and water production is used to compare both methods. ASTM D 4516 is based on normalized pressure over time for a given set of data, considers temperature only for water production but not for salt passage. The HSDM MTC is diffusion based and the HSDM considers variations in flux and recovery for salt passage. Normalization of the HSDM MTCs for temperature and pressure over time provided a universal assessment for the water and water quality for a specific diffusion controlled membrane. Assessment of water production was identical by either method, but assessment of salt passage was different. Salt passage determined by the ASTM method is dependent on actual net solute driving force, while HSDM determined salt passage is dependent on MTCs, flux, recovery, temperature, net solute driving force and changes in mass transfer over time. The HSDM MTC method of membrane evaluation is more versatile for assessment of membrane performance at varying sites and changing operation.