We evaluated sources of variance in the measurement of the surface streaming potential of flat sheet specimens of cellulose di/triacetate membranes with a commercial electrokinetic analyzer. Two measurement methods were evaluated: the instrument＇s standard operating procedure, which continuously ramps the flow past the specimen, and a modified procedure that periodically stepped the flow rate and required manual calculation of zeta potential from the archived pressure and streaming potential data. Our study indicated that the kinetics of equilibration between electrolyte, electrodes, and membrane has the greatest impact on measurement uncertainty of any method used. An optimized protocol was developed for each method and the type A uncertainty (standard deviation) for the data collected using the optimized, modified procedure was one-third the value obtained using the optimized, standard procedure.