Microfiltration comprises the largest fraction of the total membranes production in the world and is assumed to pass both salts and ionic dyes in potable water and tertiary biological wastewater treatment applications. However, inorganic and organic membranes from stainless steel, ceramic and polypropylene microtilters having pores 0.2 microns in diameter have been shown to be capable of rejecting anionic dyes and sodium nitrate from water solutions. This work has been extended to newer stainless steel membranes having smaller pore diameters of 0.1 and 0.03 microns. These membranes are able to reject ionic dyes and salt from aqueous solutions containing salts below 3000 ppm. The conductivity of the filtering solution is correlated to the Debye length which is offered as an explanation of the mechanism of rejection. The effect of pH and salt concentration on the filtration rate and color rejection is also presented.