The effects of membrane fouling and molecular weight cutoff (MWCO) on the partitioning of pectinesterase (PE) isolated from Marsh grapefruit were evaluated. Statistically higher fouling rates were observed for Marsh grapefruit extracts on 30K and 100K poly(sulfone) UF membranes than on a similar membrane with a MWCO of 10K. A simple logarithmic expression adequately described the fouling phenomena for all three UF membranes. The permeability of PE was also determined by using the same membranes and found to decrease 30−60% during processing regardless of the MWCO of the membrane. Permeability was found to decrease in a fashion similar to permeate flux. A separate logarithmic expression was developed to evaluate the rate of change of permeability on each of the membranes. About 40−50% of the total activity in the permeate of the 30K UF membrane was thermostable PE. Keywords: Polysulfone membrane; thermostable pectinesterase; citrus processing