A series of new and ultrathin membranes with thicknesses from 250 to 5000 angstroms and consisting of various polysaccharide mixed esters and ethers were tested on microporous supporting films for improved reverse osmosis treatment of municipal wastewaters. From the screening studies with secondary effluent, ultrathin membranes prepared from two polymers (out of a total of 44) looked very promising: cellulose methyl sulfonate O-propyl sulfonic acid (CMSOPSA) and cellulose acetate O-propyl sulfonic acid (CAOPSA). Long-term (150 hours) testing of the CMSOPSA membrane with secondary effluent resulted in an average water flux of 34 to 36 gfd over the last 100 hours, and rejections of 96 percent for total dissolved solids, 94 percent for ammonia, and 83 percent for total organic carbon. Preliminary membrane optimization studies showed that the reverse osmosis properties of these new membranes could be improved by closely controlling the polymer preparation procedures and the casting and annealing conditions. A preliminary economic analysis was conducted.