Humic acids of high molecular weight were removed from spent Bayer liquor by polymeric ultrafiltration membranes. Among the commercial and laboratory-cast membranes tested, Radel-R polyphenylsulfone on a polypropylene backing material was found to be the most promising candidate for this separation. However, the maximum separation of humic acids obtained at operating conditions of 50-degrees-C and 0.34 MPa, as measured by spectrophotometric analysis, was only in the 50 to 55% range. In order to explain this limited membrane separation of humic acids in spent Bayer liquor, a synthetic alkaline solution of humic acids was treated using the same membranes. These tests indicated much higher separation of humic acids (92%). Humic substances in Bayer liquor appear to be hydrolyzed and degraded to low molecular weight fractions (molecular weight < 1000 daltons) by the combined action of the strongly alkaline Bayer liquor and high digestion temperatures. These low molecular weight fractions cannot be retained by standard ultrafiltration membranes. However, some preliminary tests with laboratory-cast Radel-R nanofiltration membranes showed improved color separation (>70%) when treating spent Bayer liquor. Characterization of these new membranes, at different temperatures and pressures as well as by other factors influencing separation of humic acids from Bayer liquor, were investigated.