The deposition mechanism of colloidal gold particles of two different sizes on microfiltration (MF) membranes has been investigated under a range of pressure, electrolyte concentrations, pH, and polarization conditions. Unlike the smaller particles which gave the greater flux in the absence of stirring, the larger particles showed a greater flux under conditions of stirring. The smaller particles showed a flux significantly lower and a flux decline greater than those of the larger particles. Evidence of aggregation due to concentration polarization is confirmed using electron microscopy of membrane surface and cross sections. Addition of salt can vary the particle rejection and flux by reducing particle-particle repulsion as well as particle-membrane repulsion. The results with gold sols are compared with those previously obtained for silver sols. Colloidal gold sols appear to be more adsorptive on the membrane surface than silver particles of comparable size, resulting in lower flux and higher rejection. Charge titration of silver and gold sols indicates higher charge density on silver sols and unusual titration curves for gold due to the presence of citrate ions and destabilization of the sols.