A further investigation on the effect of solvent-system composition on microscopic structure and reverse-osmosis performance of sulphonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO)-polyethersulphone (PES) composite membranes is presented. Charged composite membranes are prepared by coating PES substrate ultrafiltration membranes with dilute solutions of hydrogen-form SPPO (SPPOH) having an ion-exchange capacity of 1.93 meq g(-1). Methanol and methanol-chloroform mixtures containing 18, 42 and 66 mass% chloroform are used as solvents for making 1.0 mass% SPPOH coating solutions. Reverse-osmosis performance of the composite membranes is investigated by measuring the membrane permeation rates and rejection for various electrolyte solutions. The effect of the solvent, used in making the coating solution, is also studied through intrinsic-viscosity measurements. The microscopic structure of the SPPOH-PES composite membranes is explored by employing a scanning electron microscope (SEM) and an atomic force microscope operated in the tapping mode (TM AFM). The reverse-osmosis performance is explained in terms of the observed TM AFM skin-layer topographs, which are in turn correlated with the intrinsic-viscosity measurements.