The fouling tendencies of seven commercial antifoam agents used with microfiltration membranes were investigated in a stirred cell. Parameters such as viscosity, oil droplet size distribution, contact angle, work of adhesion (W-a), membrane type, operating pressure, and feed concentration were examined. The results show that a silicone-based antifoam, G832, gave a significantly lower flux (less than or equal to 100 L h(-1) m(-2)) than the polypropylene glycol antifoam PPG2000 (1500-2500 L h(-1) m(-2)) over the concentrations from 0.5 to 4.0 mt L-1 at.50 kPa, 700 rpm, and 25 degrees C. The significantly lower flux attained by the antifoam G832 was attributed to a number of factors, such as its greater viscosity (1.25 x 10(-3) kg m(-1) s(-1) compared with 1.05 x 10(-3) kg m(-1) s(-1) for PPG2000), a smaller average oil droplet size (0.28 vs. 0.65 Cim) comparable to the membrane pore size distribution (0.26-0.62 Crm), a higher contact angle (98 vs. 51 degrees); and a significantly larger value of work of adhesion (26.0 vs, 0.81 mN m(-1)). The different magnitudes of membrane adsorption by these antifoam agents, as reflected by their values of work of adhesion, were also observed in the field emission scanning electronic microscopy photographs. In terms of membrane type, the inorganic Anopore alumina membrane was found to offer a higher PPG flux than the organic Millipore polyvinyl difluoride (PVDF) membranes (both hydrophilic and hydrophobic). Furthermore, the mixed-type antifoams (B5600, B426, and G832) were found to give an increased hydraulic resistance and fouling percentage with an increase in pressure or feed concentration. Conversely for PPG2000, a reduction of the hydraulic resistance and fouling percentage was observed with increases of these two parameters.