Mass transfer performance correlations are needed for spiral wound membrane module feed spacers to determine the optimal flow velocity for each spacer, to find the most cost efficient spacer, and to compare spiral wound to other modules. The mass transfer characteristics of 15 feed spacers were determined experimentally. The mass transfer correlations derived from the data indicate that a transitional flow regime exists in spacer-filled channels at the flow velocities evaluated. The use of a spacer increased the mass transfer coefficient to 1.5 to 4.0 times the coefficient obtained with an empty channel. There was only a modest increase in the performance of the spacer with the highest mass transfer coefficient when compared to the spacer with the lowest despite the fact that the friction loss characteristics of the spacers varied greatly. Attempts to define a characteristic dimension that would allow the consolidation of the mass transfer results from many spacers into a single equation were unsuccessful. Even spacers of a common design, i.e., diamond-shaped, required a unique equation to describe the mass transfer characteristics of each spacer. Ultraflo spacers from Nalle Plastics, Inc. provided the highest mass transfer coefficient per unit of pressure drop per unit length. However, the determination of the most cost efficient spacer will require the use of a detailed economic analysis.