The effects of pH and ionic strength on the performance of ceramic microfiltration membranes are evaluated. An alpha-alumina membrane, characterized by a 0.8 mum nominal pore diameter, was operated in a crossflow configuration with three types of feed waters : particle-free electrolyte solutions, dispersions of mineral colloids and solutions of humic acid. The permeate flux was affected significantly by the pH of the feed water. The experimental results are interpreted in the context of electroviscous, visco-electric, and solvation effects. These results suggest that counterions exert an influence on the permeate flux at distances substantially greater than a Debye length from the pore walls.