Effects of transmembrane pressure and membrane pore size on the microfiltration of oil-in-water emulsions are reported. A model for calculating the critical pressure required to force the entry of an oil drop into a membrane pore is formulated based on the Young-Laplace equation and serves as the basis for analyzing the rejection of emulsified materials by microfiltration membranes. This critical pressure appears to be a reasonable predictor of the operating pressure at which oil drops are no longer rejected by the membrane and their penetration leads to membrane fouling.