The oligomer fraction of poly[oxy-1,4-phenylenesulphonyl-1,4-phenyleneoxy-1,4-phenylene(1-methylethylidene)-1,4-phenylene] (Polysulphone©) with molecular weight between 450 and 900 daltons is responsible for the precipitation in concentrated solutions of polysulphone in N,N-dimethylformamide and other solvents. The precipitate is of a crystalline nature and consists mainly of the oligomers. These oligomers differ from the higher molecular weight molecules not only in crystallization properties. In a liquid-liquid phase separated system consisting of polysulphone, N,N-dimethylacetamide and water, the oligomers accumulate exclusively in the polysulphone-poor, dilute phase. The solubility parameter concept is used to illustrate the origin of this difference in behaviour. The removal of the oligomers from the dissolved polysulphone can be achieved by a crystallization process.