Good cellulose triacetate reverse osmosis membranes have been prepared by the phase separation technique. The membrane properties were influenced by the membrane casting dope composition, mostly by the dioxane/acetone ratio and the maleic acid content in the casting dope. The distinctive membrane performances were caused by different pore size distributions and various numbers of pores in the membranes skin. The improved pore size distribution analysis and the calculation of both the effective number of pores and the active part of the membranes surface area have shown a wide spectrum of the network pore sizes and usually one size group of bigger aggregate pores.