Reverse osmosis (RO) is a technology used for desalination and water purification where under high pressure (40-60 bar) water effectively passes through the semipermeable membrane, while all inorganic and organic solutes are retained as much as possible on the side of the RO membrane. Among the first materials used to prepare RO membranes was cellulose acetate (CA). Some of the main disadvantages of CA membranes are relatively low permeability, operation in a narrow neutral pH range and susceptibility to biological fouling, which significantly reduces the performance of membranes. In order to prepare membranes with improved properties, a modification of CA with the copolymer poly(methyl-methacrylate-co-dimethylaminoethyl-methacrylate, P(MMA-co-DMAEMA) was carried out. The functional comonomer DMAEMA contributes to the antibacterial properties of the copolymer, which can be further enhanced by quaternization of the tertiary amine of DMAEMA. By adding P(MMA-co-DMAEMA) it is possible to reduce the effect of biological fouling i.e., changes in separation and flow properties caused by this phenomenon. The aim of this study was to investigate the effects of adding P(MMA-co-DMAEMA) copolymer in different mass fractions and heat treatment on the morphology of asymmetric cellulose acetate membranes and their performance. The following membranes were prepared: 17 % and 20 % pure CDA, 17 % blends of 95/5 CDA and P(MMA-co-DMAEMA), and 17 % blends of 90/10 CDA and P(MMA-co-DMAEMA). The effect of the addition of the copolymer on the properties of the CDA membranes was studied in a reverse osmotic apparatus. In addition, the membranes were characterized using infrared spectroscopy (IR), contact angle measurements and a scanning electron microscope. Two series of experiments were conducted: 1) the raw membranes were subjected to pressure treatment, 2) the raw membranes were subjected to heat and pressure treatment. In the first series of experiments, the membranes prepared from a mixture of CDA and P (MMA-co-DMAEMA) obtained a higher retention coefficient for NaCl and CaCl2 than the membranes prepared from pure CDA. Despite the lower retention coefficient, the membranes prepared from pure CDA exhibited significantly higher flux values. In the second series of experiments, thermally and pressure treated membranes showed a higher retention coefficient for salts with significantly lower flux values than the pressure treated membranes.