We investigated separation characteristics of composite membranes in the process of reverse osmosis of calcium sulphate acid solutions containing Fe (III), and the impact under these conditions of an additive of acidic polyelectrolyte, sodium carboxymethylcellulose Na-CMC. It is shown that in the absence of Na-CMC the separation properties of the membranes deteriorate owing to the formation of a deposit on them. Fe(III) ions are mainly responsible for deposition. Upon the introduction to the solution filtered of the Na-CMC additive one can observe a delay and change in the nature of deposition. In this case, substantial influence of Na-CMC concentration on the deposit filtering properties was detected. We obtained data indicating stabilizing action of Na-CMC on the calcium sulphate solution containing Fe (III) ions in the acidic medium.